xilinx: zynqmp: Add support for saving variables

Enabling saving variables to MMC(FAT), NAND, SPI based on primary bootmode.
Maybe that logic can be tuned for more complicated use cases and better
tested for different bootmodes.

Tested on zcu104 to SD(FAT) and JTAG(NOWHERE).

Signed-off-by: Michal Simek <michal.simek@xilinx.com>