powerpc/8xx: Simplify CONFIG_8xx checks in Makefile

The entire 8xx directory is omitted if CONFIG_8xx is not enabled, so
within the 8xx/Makefile CONFIG_8xx is always y. So convert
obj-$(CONFIG_8xx) to the more obvious obj-y.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/8xx/Makefile

 #
 # Makefile for the PowerPC 8xx linux kernel.
 #
-obj-$(CONFIG_PPC_8xx)	  += m8xx_setup.o
+obj-y			+= m8xx_setup.o
 obj-$(CONFIG_MPC885ADS)   += mpc885ads_setup.o
 obj-$(CONFIG_MPC86XADS)   += mpc86xads_setup.o
 obj-$(CONFIG_PPC_EP88XC)  += ep88xc.o