To avoid missing arm64 specific warnings that get introduced
in this driver, allow compile-testing on all 64-bit architectures.

The only actual arm64 specific code in this driver is an open-
coded 128 bit MMIO write. On non-arm64 the same can be done
using memcpy_toio. What I also noticed is that the mmio store
(either one) is not endian-safe, this will only work on little-
endian configurations, so I also add a Kconfig dependency on
that, regardless of the architecture.
Finally, a depenndecy on CONFIG_64BIT is needed because of the
writeq().
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>