imx: mx6ul/sx: fix mmdc_ch0 clk calculation

Check "Figure 19-5. BUS clock generation" of i.MX 6SoloX Applications
Processor Reference Manual and "Figure 18-5. BUS clock generation" of
i.MX 6UltraLite Applications Processor Reference Manual. If mmdc clk
sources from pll4_main_clk(pll_audio), the calculation is wrong.

Fix mmdc_ch0 clk calculation. Also add PLL_AUDIO/VIDEO support
for decode_pll.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>