OMAP4: PRCM: move global reset function for OMAP4 to an OMAP4-specific file

Move the OMAP4 global software reset function to the OMAP4-specific
prm44xx.c file, where it belongs.  Part of the long-term process of
moving all of the direct PRCM register writes into lower-layer code.

Also add OCP barriers on OMAP2/3/4 to reduce the chance that the MPU
will continue executing while the system is supposed to be resetting
itself.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Rajendra Nayak <rnayak@ti.com>