ARM: OMAP3: Beagle: fix OPP customization and initcall ordering

After commit 24d7b40a

 (ARM: OMAP2+:
PM: MPU DVFS: use generic CPU device for MPU-SS), OPPs are registered
using an existing CPU device, not the omap_device for MPU-SS.

First, fix the board file to use get_cpu_device() as required by the
above commit, otherwise custom OPPs will be added to the wrong device.

Second, the board files OPP init is called from the its init_machine
method, and the generic CPU devices are not yet created when
init_machine is run.  Therefore OPP initialization will fail.  To fix,
use a device_initcall() for the board file's OPP customization, and
make the device_initcall board-specific by using a machine_is check.

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