powerpc/prom_init: Use IS_ENABLED()

Use IS_ENABLED() for the device tree checks, so that more code is
checked by the compiler without having to build all the different
configurations.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218113159.422821-1-mpe@ellerman.id.au