ARM: mmp: build sram driver alone

sram driver can be used by many chips besides CPU_MMP2, and so build
it alone. Also need to select MMP_SRAM for MMP_TDMA driver.

Reported-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Qiao Zhou <zhouqiao@marvell.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>