ARM: AM33xx: PRM: add support for prm_init

Added support for prm_init for AM33xx SoC. This is needed to register
SoC specific prm_ll_data for these devices.

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>