doc: Add doc for the Ingenic TCU hardware

Add documentation about the Timer/Counter Unit (TCU) present in the
Ingenic JZ47xx SoCs.

The Timer/Counter Unit (TCU) in Ingenic JZ47xx SoCs is a multi-function
hardware block. It features up to to eight channels, that can be used as
counters, timers, or PWM.

- JZ4725B, JZ4750, JZ4755 only have six TCU channels. The other SoCs all
  have eight channels.

- JZ4725B introduced a separate channel, called Operating System Timer
  (OST). It is a 32-bit programmable timer. On JZ4770 and above, it is
  64-bit.

- Each one of the TCU channels has its own clock, which can be reparented
  to three different clocks (pclk, ext, rtc), gated, and reclocked, through
  their TCSR register.
  * The watchdog and OST hardware blocks also feature a TCSR register with
    the same format in their register space.
  * The TCU registers used to gate/ungate can also gate/ungate the watchdog
    and OST clocks.

- Each TCU channel works in one of two modes:
  * mode TCU1: channels cannot work in sleep mode, but are easier to
    operate.
  * mode TCU2: channels can work in sleep mode, but the operation is a bit
    more complicated than with TCU1 channels.

- The mode of each TCU channel depends on the SoC used:
  * On the oldest SoCs (up to JZ4740), all of the eight channels operate in
    TCU1 mode.
  * On JZ4725B, channel 5 operates as TCU2, the others operate as TCU1.
  * On newest SoCs (JZ4750 and above), channels 1-2 operate as TCU2, the
    others operate as TCU1.

- Each channel can generate an interrupt. Some channels share an interrupt
  line, some don't, and this changes between SoC versions:
  * on older SoCs (JZ4740 and below), channel 0 and channel 1 have their
    own interrupt line; channels 2-7 share the last interrupt line.
  * On JZ4725B, channel 0 has its own interrupt; channels 1-5 share one
    interrupt line; the OST uses the last interrupt line.
  * on newer SoCs (JZ4750 and above), channel 5 has its own interrupt;
    channels 0-4 and (if eight channels) 6-7 all share one interrupt line;
    the OST uses the last interrupt line.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Mathieu Malaterre <malat@debian.org>
Tested-by: Artur Rojek <contact@artur-rojek.eu>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-clk@vger.kernel.org
Cc: od@zcrc.me