ARM: mmp: register audio sram bank

MMP platform has sram for the audio island;
Add the device structure for the audio sram,
and register the audio sram device at init phase.

Signed-off-by: Leo Yan <leoy@marvell.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>