arm64: dts: ti: k3-am642-sk: Add pinmux corresponding to main_uart0

Add pinmux details required for the zeroth instance of main UART.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20220516113417.3516-1-a-govindraju@ti.com