ARM: OMAP2/3: PRM: add SoC reset functions (using the CORE DPLL method)

Add SoC reset functions into the PRM code.  These functions are based
on code from mach-omap2/prcm.c.  They reset the SoC using the CORE DPLL
reset method (as opposed to one of the other two or three chip reset
methods).

Adding them here will facilitate their removal from
arch/arm/mach-omap2/prcm.c.  (prcm.c is deprecated.)

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