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/Makefile b/arch/arm/mach-aspeed/Makefile
index 1557dca..33f65b5 100644
--- a/arch/arm/mach-aspeed/Makefile
+++ b/arch/arm/mach-aspeed/Makefile
@@ -3,4 +3,4 @@
 # Copyright (c) 2016 Google, Inc
 
 obj-$(CONFIG_ARCH_ASPEED) += ast_wdt.o
-obj-$(CONFIG_ASPEED_AST2500) += ast2500/ ast2500-board.o
+obj-$(CONFIG_ASPEED_AST2500) += ast2500/