omap2 clock: use dedicated omap2_dpll_recalc() for DPLL recalc func

dpll_ck is not a clksel clock, and so it does not really fit well into
omap2_clksel_recalc().  Split off its recalc code into its own function,
omap2_dpll_recalc().

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>