xilinx: board: Add support to pick bootscr flash offset/size from DT

xilinx: board: Add support to pick bootscr flash offset/size from DT

Location of bootscript in flash can be specified via /options/u-boot DT
node by using bootscr-flash-offset and bootscr-flash-size properties.
Values should be saved to script_offset_f and script_size_f variables.
Variables are described in doc/develop/bootstd.rst as:
script_offset_f
    SPI flash offset from which to load the U-Boot script, e.g. 0xffe000

script_size_f
    Size of the script to load, e.g. 0x2000

Both of them are used by sf_get_bootflow() in drivers/mtd/spi/sf_bootdev.c
to identify bootscript location inside flash.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/60a84405f3fefabb8b48a4e1ce84431483a729f3.1693465465.git.michal.simek@amd.com