ARM: S5P6440: Remove usage of clk_p and add clk_pclk clock

The clk_p clock is of type 'struct clk' whereas on S5P6440,
the pclk is more suitable to be of type 'struct clksrc_clk'
(since pclk clock is divided version of hclk).

This patch modifies the following.

1. Adds the 'clk_pclk' clock which is of type 'struct clksrc_clk'.
2. Adds clk_pclk into the list of sysclks.
3. The clock rate 'pclk' is modified to be derived from clk_pclk.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>