ppc64: Reserve FDT memory for full elfcorehdr memory size

With --hotplug support, extra memory is added to elfcorehdr kexec
segment to accommodate additional resources during memory hotplug
events. However, the FDT is not updated with the same size, which
can lead to elfcorehdr corruption in the kdump kernel.

Fix this by updating the elfcorehdr size in the FDT memory
reservation entry to use its actual memsz instead of the current
buffer size.

Fixes: b8408c95e7c0 ("powerpc/kexec_load: add hotplug support")
Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
Cc: Simon Horman <horms@kernel.org>
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Simon Horman <horms@kernel.org>