MIPS: Alchemy: clock framework integration of onchip clocks

This patch introduces common clock framework integration for all
configurable on-chip clocks on Alchemy chips:

- 2 or 3 PLLs which generate integer multiples of the root rate 12MHz,
- 6 dividers which take one of the 3 PLLs as input and divide their
  rate by either multiples of 2 or 1 (Au1300).
- another bank of up to 6 muxes which take either one of the 6
  above dividers or one of the PLLs directly and divide their rate
  further by 1, 2, 3 or 4.
- a few other sources which are used by onchip peripherals and are
  informational.

This implementation will take the clock tree as it was set up
by boot firmware: all in-kernel boards should continue to work
without having to set up the clock tree in board code.

CLK_IGNORE_DISABLED will be removed once all drivers have been
converted.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7466/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>