powerpc/configs: add FADump awareness to skiroot_defconfig

FADump is supported on PowerNV platform. To fulfill this support, the
petitboot kernel must be FADump aware. Enable config PRESERVE_FA_DUMP
to make the petitboot kernel FADump aware.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/157062986936.23016.10146169203560084401.stgit@hbathini.in.ibm.com