ARM: clk-imx6sl: correct the pxp and epdc axi clock selections

The parent clocks of IMX6SL_CLK_PXP_AXI_SEL and IMX6SL_CLK_EPDC_AXI_SEL
clocks are not the same. So split the epdc_pxp_sels into two different
clock selections 'pxp_axi_sels' and 'epdc_axi_sels'.

Signed-off-by: Fancy Fang <chen.fang@freescale.com>
Signed-off-by: Robby Cai <R63905@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>