imx: imx8ulp: enable MU0_B clk by default

imx: imx8ulp: enable MU0_B clk by default

Enable MU0_B clk by default. When M33 image is loaded by Jlink,
the previous method not enable MU0_B clk and not able to communicate
with M33, so let's enable it by default.

And we not put it under kernel dts, because it conflicts with i.MX8QM
suspend/resume logic which requires large change.

Reviewed-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>