ram: move aspeed ram driver into drivers/ directory

to improve the maintainability.  It is more easier to modify and add
configurations of the driver in the centralized ram driver directory.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>
diff --git a/arch/arm/mach-aspeed/ast2500/Makefile b/arch/arm/mach-aspeed/ast2500/Makefile
index 4c27c8f..db70432 100644
--- a/arch/arm/mach-aspeed/ast2500/Makefile
+++ b/arch/arm/mach-aspeed/ast2500/Makefile
@@ -1,3 +1,3 @@
 obj-y += lowlevel_init.o
 obj-y += board_common.o
-obj-y += clk_ast2500.o sdram_ast2500.o
+obj-y += clk_ast2500.o