ARM: OMAP3: dpll3-m2: get rid of obsolete clksel access

Core DVFS support code is currently referencing clksel functionality, which
is wrong. The m2 divisor has been implemented as a divider clock for a long
time already, so replace the validrate check call with proper functionality.
Core DVFS is currently not supported in the kernel so verifying this patch
can be rather tricky, this should probably be fixed at some point.

Signed-off-by: Tero Kristo <t-kristo@ti.com>