ARM: dts: keystone-k2e: Add MSM RAM node

Add the RAM managed by the Multicire Shared Memory Controller (MSMC)
as a mmio-sram node. The 66AK2E SoCs have 2 MB of such memory. Any
specific MSM memory range 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: Santosh Shilimkar <ssantosh@kernel.org>