ARM: configs: keystone: Enable Generic on-chip SRAM driver

All the Keystone 2 SoCs have an on-chip RAM called the MultiCore
Shared Memory (MSM) RAM managed by the Multicore Shared Memory
Controller (MSMC) that provides faster access compared to normal
DDR.

This patch enables the Generic on-chip SRAM driver that manages
this memory in software.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>