ARM: OMAP4: PM: fix regulator name for VDD_MPU

commit 24d7b40a (ARM: OMAP2+: PM: MPU DVFS: use generic CPU device for
MPU-SS) updated the regulator name used for the MPU regulator, but only
updated OMAP3, not OMAP4.  Fix the OMAP4 name as well, otherwise CPUfreq
fails to find the MPU regulator.

Signed-off-by: Kevin Hilman <khilman@ti.com>