arm64: dts: ti: k3-am65-mcu: Add the MCU RAM node

Add the on-chip SRAM present within the MCU domain as a mmio-sram node.
The K3 AM65x SoCs have 512 KB of such memory. Any specific memory range
within this RAM needed by a software module ought to be reserved using
an appropriate child node.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>