clk: shmobile: Add MSTP clock support

MSTP clocks are gate clocks controlled through a register that handles
up to 32 clocks. The register is often sparsely populated.

Those clocks are found on Renesas ARM SoCs.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>