ARM: mvebu: enable SRAM support in mvebu_v7_defconfig

SRAM support is needed for the hardware buffer management used by the
mvneta driver.

[gregory.clement@free-electrons.com: add commit log]

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>