This patch adds at91 PMC (Power Management Controller) base support.

All at91 clocks managed by the PMC unit will use this framework.

This framework provides the following fonctionalities:
- define a new struct at91_pmc to hide PMC internals (lock, PMC memory
  mapping, irq domain, ...)
- read/write helper functions (pmc_read/write) to access PMC registers
- lock/unlock helper functions (pmc_lock/unlock) to lock/unlock access to
  pmc registers
- a new irq domain and its associated irq chip to request PMC specific
  interrupts (useful for clk prepare callbacks)

The PMC unit is declared as a dt clk provider (CLK_OF_DECLARE), and every
clk using this framework will declare a table of of_at91_clk_init_cb_t
and add it to the pmc_clk_ids table.

When the pmc dt clock setup function is called (by of_clk_init function),
it triggers the registration of every supported child clk (those matching
the definitions in pmc_clk_ids).

This patch copies at91_pmc_base (memory mapping) and at91sam9_idle
(function) from arch/arm/mach-at91/clock.c (which is not compiled if
COMMON_CLK_AT91 is enabled).

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>