ARM: OMAP2xxx: clock: move virt_prcm_set code into clkt2xxx_virt_prcm_set.c

Collect all of the virt_prcm_set-specific clocktype code into
mach-omap2/clkt2xxx_virt_prcm_set.c.  Remove its dependency on the
'sclk' and 'vclk' global variables.  Those variables will be removed
by subsequent patches.

This is part of the process of cleaning up the OMAP2xxx clock code
and preparing for the removal of the omap_prcm_restart() function.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Vaibhav Hiremath <hvaibhav@ti.com>