powerpc/Makefiles: Convert ifeq to ifdef where possible

In Makefiles if we're testing a CONFIG_FOO symbol for equality with 'y'
we can instead just use ifdef. The latter reads easily, so convert to
it where possible.

Signed-off-by: Rodrigo R. Galvao <rosattig@linux.vnet.ibm.com>
Reviewed-by: Mauro S. M. Rodrigues <maurosr@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>