OMAP3: PM: PM_MPUGRPSEL writes should use GRPSEL macros, not EN macros

Writes to the PM_*GRPSEL registers should use _GRPSEL_ macros, not _EN_ macros,
to match the TRM and guard against inadvertent error.  This patch should
not cause any functional change.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Jouni Högander <jouni.hogander@nokia.com>