powerpc/fadump: Update documentation about OPAL platform support

With FADump support now available on both pseries and OPAL platforms,
update FADump documentation with these details.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821361692.5656.11377757995827253404.stgit@hbathini.in.ibm.com