openrisc: convert makefiles to Kbuild style

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
diff --git a/arch/openrisc/cpu/Makefile b/arch/openrisc/cpu/Makefile
index 863d9b3..fc47d66 100644
--- a/arch/openrisc/cpu/Makefile
+++ b/arch/openrisc/cpu/Makefile
@@ -5,27 +5,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
-
-LIB	= $(obj)lib$(CPU).o
-
-START	= start.o
-COBJS-y	= cache.o cpu.o exceptions.o interrupts.o
-
-SRCS	:= $(START:.o=.S) $(COBJS-y:.o=.c)
-OBJS	:= $(addprefix $(obj),$(COBJS-y))
-START	:= $(addprefix $(obj),$(START))
-
-all:	$(obj).depend $(START) $(LIB)
-
-$(LIB):	$(OBJS)
-	$(call cmd_link_o_target, $(OBJS))
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
-
-#########################################################################
+extra-y	= start.o
+obj-y	= cache.o cpu.o exceptions.o interrupts.o