OMAP: Make dpll4_m4_ck programmable with clk_set_rate()

Filling the set_rate and round_rate fields of dpll4_m4_ck makes
this clock programmable through clk_set_rate().  This is needed
to give omapfb control over the dss1_alwon_fck rate.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>