OMAP3: PRM/CM: separate CM context save/restore; remove PRM context save/restore

The OMAP3 PRM module is in the WKUP powerdomain, which is always
powered when the chip is powered, so it shouldn't be necessary to save
and restore those PRM registers.  Remove the PRM register save/restore
code, which should save several microseconds during off-mode
entry/exit, since PRM register accesses are relatively slow.

While doing so, move the CM register save/restore code into
CM-specific code.  The CM module has been distinct from the PRM module
since 2430.

This patch includes some minor changes to pm34xx.c.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Tero Kristo <tero.kristo@nokia.com>
Cc: Kalle Jokiniemi <kalle.jokiniemi@digia.com>
Reviewed-by: Kevin Hilman <khilman@deeprootsystems.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Rajendra Nayak <rnayak@ti.com>