davinci: DA850/OMAP-L138: avoid using separate initcall for initializing regulator

Using a device_initcall() for initializing the voltage regulator
on DA850 is not such a good idea because it gets called for all
platforms - even those who do not have a regulator implemented.
This leads to a big fat warning message during boot-up when
regulator cannot be found.

Instead, tie initialization of voltage regulator to cpufreq init.
Define a platform specific init call which in case of DA850 gets
used for initializing the regulator. On other future platforms it
can be used for other purposes.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>