armv7R: dts: k3: am654: Add MCU_UART0 related definitions

Although we currently use the MAIN_UART0 for R5 SPL console output there
are cases where we require access to the MCU_UART0 as well for example in
case of UART-based Y-Modem boot. To support these scenarios add related
DTS definitions to be able to use that UART early on.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>