powerpc: make syntax for FADump config options in kernel/Makefile readable

arch/powerpc/kernel/fadump.c file needs to be compiled in if 'config
FA_DUMP' or 'config PRESERVE_FA_DUMP' is set. The current syntax
achieves that but looks a bit odd. Fix it for better readability.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/157063484064.11906.3586824898111397624.stgit@hbathini.in.ibm.com