xilinx: board: Add support to pick bootscr address from DT

xilinx: board: Add support to pick bootscr address from DT

The bootscript is expected at a default address specific to each
platform.
When high speed memory like Programmable Logic Double Data Rate RAM
(PL DDR RAM) or Higher Bandwidth Memory RAM (HBM) is used the boot.scr
may be loaded at a different offset. The offset needs to be set through
setenv. Due to the default values in some cases the boot.scr is falling
in between the kernel partition.

The bootscript address or the bootscript offset is fetched directly from
the DT and updated in the environment making it easier for automated
flows.

Signed-off-by: Algapally Santosh Sagar <santoshsagar.algapally@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/fac7020b31e1f150b021d666f0d588579ea671ad.1693465140.git.michal.simek@amd.com