arm64: Add initial DTS for APM X-Gene Storm SOC and APM Mustang board

This patch adds initial DTS files required for APM Mustang board.

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>