ARM: S5P6440: Remove usage of clk_p_low and add clk_pclk_low clock

The pclk_low clock is of type 'struct clk' whereas on S5P6440,
the pclk_low clock is more suitable to be of type 'struct clksrc_clk'
(since pclk_low clock is a divided clock of hclk_low clock).

This patch modifies the following.

1. Removes the definition and usage of clk_p_clk clock.
2. Adds the clk_pclk_low clock of type 'struct clksrc_clk' clock.
3. Adds clk_pclk_low to the list of system clocks.
4. The clock rate of pclk_low is derived from the clk_pclk_low clock.

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>