arm64: dts: Add more serial port nodes in APM X-Gene device tree

APM X-Gene Storm SoC supports 4 serial ports. This patch adds device nodes
for serial ports 1 to 3 (a device node for serial port 0 is already present
in the dts file).
This patch also sets the compatible property of serial nodes to "ns16550a".

Signed-off-by: Vinayak Kale <vkale@apm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>