sifive: fu540: Add boot flash script offset, size

HiFive-Unleashed-A00 has SPI flash with 32MiB size.
So, let's use the script offset at the end of 4K.
This way it cannot overlap any offsets being used
by software components in flash layout.

So, SF distrocmd will pick the script at desired
script address and run.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>