xilinx: common: Fix boot script address

Currently u-boot supports addresses upto 39-bits only. If anybody
wants to use addresses of more than 39-bits in Linux they will have
a separate memory node in DT. In such cases they will have multiple
memory nodes.
Currently u-boot selects and runs on lower memory bank region.
But bootscript is being loaded on dram bank 0, where dram bank 0 will
point to 1st memory node in DT. If first memory node is mentioned as
higher ddr(>39-bits address) then u-boot cannot load the bootscript.
So fix this issue by setting bootscript address within the lower memory
bank region.

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