ARM: OMAP4: PRM: make PRCM interrupt handler related functions static

These are not needed outside the PRM driver, so make them static and
remove the prototypes from the public header.

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>