powerpc/fadump: use FADump instead of fadump for how it is pronounced

fadump is pronounced f-a-dump. Update documentation accordingly. Also,
update how fadump_region contents look like with recent changes.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821339317.5656.15852294223821732082.stgit@hbathini.in.ibm.com