ARM: AM33xx: PRM: make direct register access functions static

These should not be accessed outside driver, thus removed the APIs
from the header file and made the implementation static.

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>