arch: arm: mach-k3: am642_init: bring back MCU_PADCFG_MMR1 unlock

Without this register unlock it is not possible to configure the
pinmux used for mcu spi0.

Fixes: 92e46092 ("arch: arm: mach-k3: am642_init: Probe ESM nodes")
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Nishanth Menon <nm@ti.com>