ARM: keystone: Make PM bus ready before populating platform devices

Keystone PM bus makes use of generic PM clock core backend. Since
generic PM clock core uses platform bus notifiers to track events like
ADD_DEVICE/DEL_DEVICE and to fill clock lists per each device, we need
to initialise Keystone PM domains before the platform devices have been
created.

Hence, fix it by moving keystone_pm_runtime_init() before platform
devices have been populated.

Reported-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>