zynqmp: Add support to provide silicon id through SMC

Add support to provide silicon id to non-secure
software through SMC.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>

[ sb
Move zynqmp_get_silicon_id outside of compile guards to avoid build
errors.
]

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>