dts: arm: ti: Add dtsi for AM64x M4

This file is basically a copy of the AM62x M4 dtsi but an
additional mcu_uart1 interface.

The internal clock frequency feeded into the UART IP is
96 MHz instead of 48 MHz, which is different to the AM62x.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>