OMAP2xxx clock: move the DVFS virtual clock code into mach-omap2/clkt2xxx_virt_prcm_set.c

OMAP2xxx clock: move the DVFS virtual clock code into mach-omap2/clkt2xxx_virt_prcm_set.c

Move the DVFS virtual clock functions from clock2xxx.c to
mach-omap2/clkt2xxx_virt_prcm_set.c.  This is intended to make the
clock code easier to understand, since all of the functions needed to
manage the virt_prcm_set clock are now located in their own file,
rather than being mixed with other, unrelated functions.

Clock debugging is also now more finely-grained, since the DEBUG macro
can now be defined for the virt_prcm_set clock alone.  This should
reduce unnecessary console noise when debugging.

Also, if at some future point the mach-omap2/ directory is split into
OMAP2/3/4 variants, this clkt file can be placed in the mach-omap2xxx/
directory, rather than shared with other chip types that don't use
this clock type.

Thanks to Alexander Shishkin <virtuoso@slind.org> for his comments.
Thanks also to Kevin Hilman <khilman@deeprootsystems.com> for finding
and fixing a bug with the CONFIG_CPU_FREQ portion of this patch.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Richard Woodruff <r-woodruff2@ti.com>
Cc: Alexander Shishkin <virtuoso@slind.org>
Cc: Kevin Hilman <khilman@deeprootsystems.com>