omap2: convert pm.c to use new PRCM functions and symbolic constants

Replace magic numbers, prcm-regs.h-sourced defines, and local defines
with the new PRCM register and register bit defines.  Also convert
register read/write code to use {cm,prm}_{read,write}_{,mod}_reg().
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>