sifive: fu540: Add U-Boot proper sector start

Add U-Boot proper sector start offset for SiFive FU540.
This value is based on the partition layout supported
by SiFive FU540.

u-boot.itb need to write on this specific offset so-that
the SPL will retrieve it from here and load.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>