soc: xilinx: zynqmp: Add soc_xilinx_zynqmp driver

soc_xilinx_zynqmp driver allows identification of family & revision
of zynqmp SoC. This driver is selected by CONFIG_SOC_XILINX_ZYNQMP.
Add this config to xilinx_zynqmp_virt_defconfig file.
Probe this driver using platdata U_BOOT_DEVICE structure which is
specified in mach-zynqmp/cpu.c.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Reviewed-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>