IRQCHIP: bcm7120-l2: Refactor driver for arbitrary IRQEN/IRQSTAT offsets

Currently the driver assumes that REG_BASE+0x00 is the IRQ enable mask,
and REG_BASE+0x04 is the IRQ status mask.  This is true on BCM3384 and
BCM7xxx, but it is not true for some of the controllers found on BCM63xx
chips.  So we will change a couple of key assumptions:

 - Don't assume that both the IRQEN and IRQSTAT registers will be
   covered by a single ioremap() operation.

 - Don't assume any particular ordering (IRQSTAT might show up before
   IRQEN on some chips).

 - For an L2 controller with >=64 IRQs, don't assume that every
   IRQEN/IRQSTAT pair will use the same register spacing.

This patch changes the "plumbing" but doesn't yet provide a way for users
to instantiate a controller with arbitrary IRQEN/IRQSTAT offsets.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: jaedon.shin@gmail.com
Cc: abrestic@chromium.org
Cc: tglx@linutronix.de
Cc: jason@lakedaemon.net
Cc: jogo@openwrt.org
Cc: computersforpeace@gmail.com
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8841/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>