ARM: support big-endian for the virt architecture

This architecture, used for running in QEMU, runs just fine when
compiled in big-endian mode. So enable it. This is enabled in exactly
the same way that it is for other platforms (such as vexpress) that also
support big-endian mode in QEMU successfully.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>