arm64: defconfig: enable Generic on-chip SRAM driver

Most of the ARM SoCs have small SRAM memory used for various purposes
that require faster access to data compared to normal DDR.

This patch enables the Generic on-chip SRAM driver on ARM64.

Cc: Kevin Hilman <khilman@kernel.org>
Cc: Olof Johansson <olof@lixom.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>