arm64: Add Kconfig option for APM X-Gene SOC family

This patch adds arm64/Kconfig option for APM 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>