ARM: dts: keystone-k2l-evm: Remove MSM memory from dspmem node

Remove the current MSM RAM memory being defined and used within
the dspmem node. The keystone_dsp_mem driver will be enhanced
to use the MSM RAM regions by means of defining appropriate
child nodes under the MSM SRAM node.

While at this, also fix the missing unit name on the dspmem
node.

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