arm: dts: Add DTS description for MMC2 am33xx devices

This code has been ported from Linux v5.16.9 arch/arm/boot/dts/am33xx.dtsi
file to allow correct usage of MMC2 controller in U-Boot.

This IP block is a bit specific as it is connected to L3 interconnect bus,
whereas mmc[01] are connected to L4.

Proper configuration of this block (including providing clock) is handled
in ti-sysc.c driver.

Signed-off-by: Lukasz Majewski <lukma@denx.de>