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>