arm: dts: k3-j721e-r5-common-proc-board: Disable power-domains for mcu uart

mcu uart will be used during uart boot for loading sysfw.itb. Since
sysfw is not yet available during uart load, power-domain cannot be
enabled. We need to rely on ROM for doing that, so disable power-domains
and clocks for mcu uart. Also fix the mcu uart frequency.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>