[ARM] omap: handle RATE_CKCTL via .set_rate/.round_rate methods

It makes no sense to have the CKCTL rate selection implemented as a flag
and a special exception in the top level set_rate/round_rate methods.
Provide CKCTL set_rate/round_rate methods, and use these for where ever
RATE_CKCTL is used and they're not already overridden.  This allows us
to remove the RATE_CKCTL flag.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>