phy: xgene: Depend on ARCH_XGENE rather than plain arm64

phy: xgene: Depend on ARCH_XGENE rather than plain arm64

The X-Gene PHY driver is unlikely to be useful on a kernel without general
X-Gene support enabled but currently only depends on arm64 rather than the
specific platform support. Narrow the dependency to ARCH_XGENE like we do
for other X-Gene specific drivers to ensure that users who have configured
down the set of platforms enabled don't see the option.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230221-phy-build-deps-v1-1-7091bcbd16b0@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>