clk: keystone: Add gate control clock driver

Add the driver for the clock gate control which uses PSC (Power Sleep
Controller) IP on Keystone 2 based SOCs. It is responsible for enabling and
disabling of the clocks for different IPs present in the SoC.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>