xilinx: Add DDR base address to bootscript address

Add ram base address to scriptaddr env variable to make boot
script address to be a valid address when ddr base address changes.

This works properly if the first memory region is the region where uboot
runs. Also the solution was taken in respect of a lot of jtag script
putting u-boot script to certain address. For standard cases
bd->bi_dram[0].start is 0 all the time. Only for systems with DDR placed
out of this location it does calculation.

This is not the best solution and should be done differently in future but
enough for now till we don't have full solution ready yet.

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