ARM: S3C24XX: add get_rate for clk_p on S3C2416/2443

Currently the rate of clk_p is calculated once and set through the
s3c24xx_setup_clocks call. As the clk_p is a child of clk_h we can
evaluate its divider and calculate the rate in get_rate.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>