ARM: keystone: Merge PM function into main support file

The one function left in this file does not justify a whole extra
file nor the keystone.h support file used to forward declare that
function. Merge this into the main keystone.c file.

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20230726160441.101566-2-afd@ti.com


Signed-off-by: Nishanth Menon <nm@ti.com>