ARM: keystone: add PM domain support for clock management

Add runtime PM core support to Keystone SOCs by using the pm_clk
infrastructure of the PM core. Patch is based on Kevin's pm_domain
work on DaVinci SOCs.

Keystone SOC doesn't have depedency to enable clocks in early
in the boot and hence the clock and PM domain initialisation is done
at subsys_init() level.

Cc: Kevin Hilman <khilman@linaro.org>

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>