xilinx: zynqmp: Set modeboot env variable in eMMC bootmode

Set environment variable modeboot to "emmcboot" in case of eMMC boot
mode.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/c61231e4b8c6118862dfc82e923211637bf29991.1631688736.git.michal.simek@xilinx.com