ARM: AM33xx: PRM: move global warm reset implementation to driver

Moved the implementation from am33xx-restart.c to the prm33xx.c file to
isolate the PRM register accesses to be private for PRM driver.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>