ARM: OMAP2+: PM: MPU DVFS: use generic CPU device for MPU-SS

ARM: OMAP2+: PM: MPU DVFS: use generic CPU device for MPU-SS

Currently, a dummy omap_device is created for the MPU sub-system so
that a device node exists for MPU DVFS.  Specifically, for the
association of MPU OPPs to a device node, and so that a voltage
regulator can be mapped to a device node.

For drivers to get a handle to this device node, an OMAP-specific API
has been used.  However, the kernel already has device nodes for the
CPU(s) in the system, so we can use those instead of an OMAP-specific
dummy device and then drivers (like OMAP CPUfreq) can use generic
APIs.

To use the existing CPU device nodes, modify the OPP creation and
regulator registration to use the CPU0 device node for registraion.

NOTE: this patch always uses CPU0 as the device node.  On all
      OMAPs today, MPU DVFS scales all CPUs together, so this will
      not be a problem, but this assumption will need to be changed
      if independently scalable CPUs are introduced.

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