xilinx: Setup bootm variables

xilinx: Setup bootm variables

On system with PL DDR which is placed before PS DDR in DT
env_get_bootm_size() and env_get_bootm_low() without specifying bootm_low
and bootm_size variables are taking by default gd->bd->bi_dram[0].start and
gd->bd->bi_dram[0].size. As you see 0 means bank 0 which doesn't need to be
PS ddr and even can be memory above 39bit VA which is what U-Boot supports
now.
That's why setup bootm variables based on ram_base/ram_size setting to make
sure that boot images are placed to the same location as U-Boot is placed.
This location should be by default location where OS can boot from.

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