Add the omap3_core_dpll_m2_set_rate() function to the OMAP3 clock code,
which calls into the SRAM function omap3_sram_configure_core_dpll() to
change the CORE DPLL M2 divider.  (SRAM code is necessary since rate changes
on clocks upstream from the SDRC can glitch SDRAM accesses.)

Use this function for the set_rate function pointer in the dpll3_m2_ck
struct clk.  With this function in place, PM/OPP code should be able to
alter SDRAM speed via code similar to:

      clk_set_rate(&dpll3_m2_ck, target_rate).
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>