clk: imx: imx8mn: fix pll mux bit

clk: imx: imx8mn: fix pll mux bit

pll BYPASS bit should be kept inside pll driver for glitchless freq
setting following spec. If exposing the bit, that means pll driver and
clk driver has two paths to touch this bit, which is wrong.

So use EXT_BYPASS bit here.

And drop uneeded set parent, because EXT_BYPASS default is 0.

Suggested-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lkml.kernel.org/r/1568043491-20680-5-git-send-email-peng.fan@nxp.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>