powerpc/fadump: make use of memblock's bottom up allocation mode

Earlier, memblock_find_in_range() was not used to find the memory to
be reserved for FADump as bottom up allocation mode was not supported.
But since commit 79442ed1 ("mm/memblock.c: introduce bottom-up
allocation mode") bottom up allocation mode is supported for memblock.
So, use it to find the memory to be reserved for FADump.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821364211.5656.14336025460336135194.stgit@hbathini.in.ibm.com