ARM: OMAP4+: PM: Consolidate OMAP4 PM code to re-use it for OMAP5

OMAP5 has backward compatible PRCM block and it's programming
model is mostly similar to OMAP4. Same is going to be maintained
for future OMAP4 based SOCs. Hence consolidate the OMAP4 power
management code so that it can be re-used on OMAP5 and later devices.

While at it, update the kernel-doc for omap4_pm_init().

Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>