ARM: OMAP2+: PRCM: remove obsolete prcm.[ch]

arch/arm/mach-omap2/prcm.c and arch/arm/plat-omap/include/plat/prcm.h
are now completely unused and can be removed.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Vaibhav Hiremath <hvaibhav@ti.com>