zynqmp: Set sd boot device number to sdbootdev

zynqmp: Set sd boot device number to sdbootdev

In case of multiple sd devices to identify proper sd boot device
assign "sdbootdev" variable with device sequence number. While
auto-booting "sdboot" picks appropriate sd device from "sdbootdev".
	This fix is for old way auto-boot process, it will be
deprecated in 2020.1 & all the functionality will be removed.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Raju Kumar Pothuraju <raju.kumar-pothuraju@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>