[ARM] 3628/1: S3C24XX: add get_rate call to struct clk

Patch from Ben Dooks

Add a get_rate call to allow an given clock
to over-ride the clk_get_rate() call.

This provides support for clocks which rely on
division of their parent to correctly report
their frequency when the parent can also change.

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