zynq: Add support to find bootmode

Added support to find the bootmodes by reading
slcr bootmode register. this can be helpful to
autoboot the configurations w.r.t a specified bootmode.

Added this functionality on board_late_init as it's not
needed for normal initializtion part.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>