arm64: Enable APM X-Gene SOC family in the defconfig

This patch enables APM X-Gene SOC family in the defconfig. It also enables 8250 serial driver needed by X-Gene SOC family.

Signed-off-by: Kumar Sankaran <ksankaran@apm.com>
Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>