powerpc: Add support code for kexec_file_load()

This patch adds the support code needed for implementing
kexec_file_load() on powerpc.

This consists of functions to load the ELF kernel, either big or little
endian, and setup the purgatory enviroment which switches from the first
kernel to the second kernel.

None of this code is built yet, as it depends on CONFIG_KEXEC_FILE which
we have not yet defined. Although we could define CONFIG_KEXEC_FILE in
this patch, we'd then have a window in history where the kconfig symbol
is present but the syscall is not, which would be awkward.

Signed-off-by: Josh Sklar <sklar@linux.vnet.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>