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>

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

drivers/ram/Makefile

@@ -14,6 +14,7 @@ obj-$(CONFIG_ARCH_ROCKCHIP) += rockchip/
 
 obj-$(CONFIG_K3_AM654_DDRSS) += k3-am654-ddrss.o
 obj-$(CONFIG_ARCH_MEDIATEK) += mediatek/
+obj-$(CONFIG_ARCH_ASPEED) += aspeed/
 obj-$(CONFIG_K3_J721E_DDRSS) += k3-j721e/
 
 obj-$(CONFIG_IMXRT_SDRAM) += imxrt_sdram.o

drivers/ram/aspeed/Makefile

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0+
+#
+obj-$(CONFIG_ASPEED_AST2500) += sdram_ast2500.o