cosmetic: aspeed: ast2500: Rename board file

Rename the ast2500-board.c to board_common.c and
place the renamed file under the ast2500 folder.

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>
diff --git a/arch/arm/mach-aspeed/ast2500/Makefile b/arch/arm/mach-aspeed/ast2500/Makefile
index 2e9e15d..4c27c8f 100644
--- a/arch/arm/mach-aspeed/ast2500/Makefile
+++ b/arch/arm/mach-aspeed/ast2500/Makefile
@@ -1,2 +1,3 @@
 obj-y += lowlevel_init.o
+obj-y += board_common.o
 obj-y += clk_ast2500.o sdram_ast2500.o