ARM: OMAP2xxx: clock: remove global 'dclk' variable

Remove the global 'dclk' variable, instead replacing it with a
variable local to the dpllcore clock type C file.  This removes some
of the special-case code surrounding the OMAP2xxx clock init.

This patch is a prerequisite for the removal of the
omap_prcm_restart() code from arch/arm/mach-omap2/prcm.c.  It also
cleans up some special-case OMAP2xxx clock code in the process.

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