TEMP: ARM: dts: k2g-evm: Add dspmem node

Add a dspmem node on the K2 Galileo EVM board. The node
allows the entire MSM RAM (1 MB) to be mapped to userspace
to enable the Keystone Multi Proc Manager (MPM) based
usecases.

Signed-off-by: Suman Anna <s-anna@ti.com>