powerpc: Update Kconfig + Makefile to prepare for server doorbells

Move the rule to build doorbell support out of the Makefile and into a
new Kconfig boolean that platforms can select.

We will add doorbell support to pseries as well in the next patch.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Tested-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 8f61934..44fbbea 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -75,8 +75,8 @@
 obj64-$(CONFIG_HIBERNATION)	+= swsusp_asm64.o
 obj-$(CONFIG_MODULES)		+= module.o module_$(CONFIG_WORD_SIZE).o
 obj-$(CONFIG_44x)		+= cpu_setup_44x.o
-obj-$(CONFIG_PPC_FSL_BOOK3E)	+= cpu_setup_fsl_booke.o dbell.o
-obj-$(CONFIG_PPC_BOOK3E_64)	+= dbell.o
+obj-$(CONFIG_PPC_FSL_BOOK3E)	+= cpu_setup_fsl_booke.o
+obj-$(CONFIG_PPC_DOORBELL)	+= dbell.o
 obj-$(CONFIG_JUMP_LABEL)	+= jump_label.o
 
 extra-y				:= head_$(CONFIG_WORD_SIZE).o