arm926ejs: convert makefiles to Kbuild style

Note1:
In arch/arm/cpu/arm926ejs/spear/Makefile
START := start.o
was changed
extra-$(CONFIG_SPL_BUILD) := start.o
because spear/start.o is only used for SPL.

Note2:
START := start.o
was missing from arch/arm/cpu/arm926ejs/mxs/Makefile.
This commit simply adds
extra-$(CONFIG_SPL_BUILD) := start.o

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/arm/cpu/arm926ejs/orion5x/Makefile b/arch/arm/cpu/arm926ejs/orion5x/Makefile
index 8f36dc0..546ebcb 100644
--- a/arch/arm/cpu/arm926ejs/orion5x/Makefile
+++ b/arch/arm/cpu/arm926ejs/orion5x/Makefile
@@ -9,31 +9,10 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
-
-LIB	= $(obj)lib$(SOC).o
-
-COBJS-y	= cpu.o
-COBJS-y	+= dram.o
-COBJS-y	+= timer.o
+obj-y	= cpu.o
+obj-y	+= dram.o
+obj-y	+= timer.o
 
 ifndef CONFIG_SKIP_LOWLEVEL_INIT
-SOBJS	:= lowlevel_init.o
+obj-y	+= lowlevel_init.o
 endif
-
-SRCS	:= $(SOBJS:.o=.S) $(COBJS-y:.o=.c)
-OBJS	:= $(addprefix $(obj),$(SOBJS) $(COBJS-y))
-
-all:	$(obj).depend $(LIB)
-
-$(LIB):	$(OBJS)
-	$(call cmd_link_o_target, $(OBJS))
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
-
-#########################################################################