OMAP3: control/PRCM: move CONTROL_PADCONF_SYS_NIRQ save/restore to SCM code

For some reason, the PRCM context save/restore code also saves and
restores a single System Control Module register,
CONTROL_PADCONF_SYS_NIRQ.  This is probably just an error -- the
register should be handled by SCM code -- so this patch moves it
there.

If this register really does need to be saved and restored before the
rest of the PRCM registers, the code to do so should live in the SCM
code, and the PM code should call this separate function.  This
register pertains to devices with a stacked modem, so this patch is
unlikely to affect most OMAP devices out there.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.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>