imx8: scu: use dedicated MU for SPL

SPL runs in EL3 mode, except MU0_A, others are not powered on,
and could not be used. However normal U-Boot use MU1_A, so we
could not reuse the one in dts. And we could not replace the one
in dts with MU0_A, because MU0_A is reserved in secure world.

Signed-off-by: Peng Fan <peng.fan@nxp.com>