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>
diff --git a/arch/arm/mach-mmp/Kconfig b/arch/arm/mach-mmp/Kconfig
index ebdda83..ebdba87 100644
--- a/arch/arm/mach-mmp/Kconfig
+++ b/arch/arm/mach-mmp/Kconfig
@@ -136,4 +136,7 @@
 	help
 	  Enables support for OTG controller which can be switched to host mode.
 
+config MMP_SRAM
+	bool
+
 endif