ARM: OMAP2+: PRM: initialize some PRM functions early

Some PRM functions will need to be called by the hwmod code early in
kernel init.  To handle this, split the PRM initialization code into
early and late phases.  The early init is handled via mach-omap2/io.c,
while the late init is handled by subsys_initcall().

Signed-off-by: Paul Walmsley <paul@pwsan.com>