ARM: OMAP4+: PRM: Move function prototypes to common header for re-use

OMAP5 reuses OMAP4 PRM IP block which lets us re-use PRM functions.
So move the function prototypes from prm44xx.h to prm44xx_54xx.h
header. The suggestion came from Paul Walmsley as part of the
OMAP5 data file review.

This is preparatory  patch to add OMAP5 PRM data file.

Cc: Paul Walmsley <paul@pwsan.com>
Acked-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>