omap2 clock: generalize initial clock rate setup: recalculate_root_clocks()

Add a new clock framework function, recalculate_root_clocks(), that
recalculates all root clocks (i.e., clocks with no parent clock).
Call this function in omap2_clk_arch_init(), rather than manually
propagating specific clocks.  Add propagate_rate() recalcs to func_32k_ck
and osc_ck.

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