powerpc: convert makefiles to Kbuild style

Note:
arch/powerpc/cpu/mpc8260/Makefile is originally like follows:

    ---<snip>---
    START   = start.o kgdb.o
    COBJS   = traps.o serial_smc.o serial_scc.o cpu.o cpu_init.o speed.o \
    ---<snip>---
    COBJS-$(CONFIG_ETHER_ON_SCC) = ether_scc.o
    ---<snip>---
    $(LIB): $(OBJS)
            $(call cmd_link_o_target, $(OBJS) $(obj)kgdb.o)

The link rule `$(call cmd_link_o_target, $(OBJS) $(obj)kgdb.o)'
is weird.
kbdg.o is not included in $(OBJS) but linked into $(LIB)
and $(LIB) is not dependent on kgdb.o.
(Broken dependency tracking)

So,
    START   = start.o kgdb.o
shoud have been
    START   = start.o
    SOBJS   = kgdb.o

That is why this commit adds kgdb.o to obj-y, not to extra-y.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
diff --git a/arch/powerpc/cpu/mpc86xx/Makefile b/arch/powerpc/cpu/mpc86xx/Makefile
index 04271b0..bcb786d 100644
--- a/arch/powerpc/cpu/mpc86xx/Makefile
+++ b/arch/powerpc/cpu/mpc86xx/Makefile
@@ -8,42 +8,20 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
+extra-y	= start.o
+extra-y	+= traps.o
 
-LIB	= $(obj)lib$(CPU).o
+obj-y += cache.o
+obj-$(CONFIG_MP) += release.o
 
-SSTART	= start.o
-CSTART	= traps.o
-
-SOBJS-y += cache.o
-SOBJS-$(CONFIG_MP) += release.o
-
-COBJS-y	+= cpu.o
-COBJS-y	+= cpu_init.o
+obj-y	+= cpu.o
+obj-y	+= cpu_init.o
 # 8610 & 8641 are identical w/regards to DDR
-COBJS-$(CONFIG_MPC8610) += ddr-8641.o
-COBJS-$(CONFIG_MPC8641) += ddr-8641.o
-COBJS-$(CONFIG_OF_LIBFDT) += fdt.o
-COBJS-y	+= interrupts.o
-COBJS-$(CONFIG_MP) += mp.o
-COBJS-$(CONFIG_MPC8610) += mpc8610_serdes.o
-COBJS-$(CONFIG_MPC8641) += mpc8641_serdes.o
-COBJS-y	+= speed.o
-
-SRCS	:= $(START:.o=.S) $(SOBJS-y:.o=.S) $(COBJS-y:.o=.c)
-OBJS	:= $(addprefix $(obj),$(SOBJS-y) $(COBJS-y))
-START	:= $(addprefix $(obj),$(SSTART) $(CSTART))
-
-all:	$(obj).depend $(START) $(LIB)
-
-$(LIB):	$(OBJS)
-	$(call cmd_link_o_target, $(OBJS))
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
-
-#########################################################################
+obj-$(CONFIG_MPC8610) += ddr-8641.o
+obj-$(CONFIG_MPC8641) += ddr-8641.o
+obj-$(CONFIG_OF_LIBFDT) += fdt.o
+obj-y	+= interrupts.o
+obj-$(CONFIG_MP) += mp.o
+obj-$(CONFIG_MPC8610) += mpc8610_serdes.o
+obj-$(CONFIG_MPC8641) += mpc8641_serdes.o
+obj-y	+= speed.o