ENGR00329352-1 regmap: regmap-mmio: make clk_id optionally when getting clock

ENGR00329352-1 regmap: regmap-mmio: make clk_id optionally when getting clock

According to clock framework, the clk_id could be NULL when getting clock.
But current code relies on a non null clk_id to get clock.
Changing the code to allow a null clk_id to get clock to make it more
reasonable to use.
And the regmap_mmio_gen_context will try to get clock by default but ignore
error if not finding the clock in case some regmap access not reply on
a specific clock.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
(cherry picked from commit e94692d02c66c87c246af3b7c311d02a2d04c739)