[POWERPC] kdump: Support kernels having 64k page size.

This is required to generate proper core files using kdump on ppc64.

Create a backup region of 64K size irrespective of the PAGE SIZE.
At present 32K was used as backup size. In the case of 64K page size,
second PT_LOAD segments starts at 32K and the first one is not page
aligned.  __ioremap() (crash_dump.c) fails if pfn = 0 which is the
case for the second PT_LOAD segment. This is not an issue for 4K page
size because the the first page (32K backup) is copied to second
kernel memory and thus referencing with the second kernel pfn.

Signed-off-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>