crashdump-ppc64: crashkernel-base and crashkernel-size are big-endian

When reading the device-tree exported crashkernel-base and
crashkernel-size, their values should be converted from big-endian to the
CPU byte order.

These is the output of running kexec --print-ckr-size on a little-endian
ppc64 box.

$ kexec --print-ckr-size
137438953472
$ kexec --print-ckr-size
536870912

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Simon Horman <horms@verge.net.au>