clk: Add clock driver for mb86s7x

The CRG11 clock controller is managed by remote f/w.
This driver simply maps Linux CLK ops onto mailbox api.

Signed-off-by: Andy Green <andy.green@linaro.org>
Signed-off-by: Vincent Yang <vincent.yang@socionext.com>
Signed-off-by: Tetsuya Nuriya <nuriya.tetsuya@socionext.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>