ARM: OMAP2+: board files: use SoC-specific system restart functions

Modify the board files to use the SoC-specific system restart
functions.  At this point it's possible to remove omap_prcm_restart()
from mach-omap2/prcm.c.

While removing the prototypes for the now-unused restart functions, clean
up a few more obsolete prototypes in mach-omap2/clock.h.

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