Some prm and cm registers read/write and status functions
are built only for some custom OMAP2+ builds and are stubbed
in header files for other builds under ifdef statements.
But this results in adding new CONFIG_ARCH_OMAPXXX
checks when SOCs are added in the future. So move them
to a common place for OMAP2+ and make them 'weak' implementations.

This way no new ifdefs would be required in the future and also
cleans up the existing code.
Signed-off-by: NR Sricharan <r.sricharan@ti.com>
[paul@pwsan.com: unsplit quoted strings; moved PRM functions to
 mach-omap2/prm_common.c; resolved sparse warnings]
Signed-off-by: NPaul Walmsley <paul@pwsan.com>