[ARM] 4777/1: S3C24XX: Ensure clk_set_rate() checks the set_rate method for the clk

Add checks for clk_set_rate() and ensure that we do not allow set_rate
to be called for a clock that does not have it defined. Add default
methods for fclk, hclk, pclk and mpll.

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