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/mpc8260/Makefile b/arch/powerpc/cpu/mpc8260/Makefile
index d87eff6..83adc4c 100644
--- a/arch/powerpc/cpu/mpc8260/Makefile
+++ b/arch/powerpc/cpu/mpc8260/Makefile
@@ -5,33 +5,9 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
-
-LIB	= $(obj)lib$(CPU).o
-
-START	= start.o kgdb.o
-COBJS	= traps.o serial_smc.o serial_scc.o cpu.o cpu_init.o speed.o \
+extra-y	= start.o
+obj-y	= traps.o serial_smc.o serial_scc.o cpu.o cpu_init.o speed.o \
 	  interrupts.o ether_fcc.o i2c.o commproc.o \
-	  bedbug_603e.o pci.o spi.o
-
-COBJS-$(CONFIG_ETHER_ON_SCC) = ether_scc.o
-
-COBJS	+= $(COBJS-y)
-
-SRCS	:= $(START:.o=.S) $(SOBJS:.o=.S) $(COBJS:.o=.c)
-OBJS	:= $(addprefix $(obj),$(SOBJS) $(COBJS))
-START	:= $(addprefix $(obj),$(START))
-
-all:	$(obj).depend $(START) $(LIB)
-
-$(LIB):	$(OBJS)
-	$(call cmd_link_o_target, $(OBJS) $(obj)kgdb.o)
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
+	  bedbug_603e.o pci.o spi.o kgdb.o
 
-#########################################################################
+obj-$(CONFIG_ETHER_ON_SCC) += ether_scc.o