imx: mx6sx: select OSC as uart's clk parent

As M4 is sourcing UART clk from OSC, to make UART work
when M4 is enabled, need to select OSC as clk parent,
24M OSC is enough for debug UART in uboot.

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