powerpc/kexec: Fix boundary case for book-e kexec memory limits

The KEXEC_*_MEMORY_LIMITs are inclusive addresses.  We define them as
2Gs as that is what we allow mapping via TLBs.  However, this should be
2G - 1 to be inclusive, otherwise if we have >2G of memory in a system
we fail to boot properly via kexec.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>