visionfive2-firmware: Update U-Boot version

Update U-Boot version to hash from JH7110_VF2_6.1_v5.10.3 tag.
Also change to "nobranch=1", since the starfive U-Boot repo does
not seem to keep releases on the development branches, causing builds
to fail when the referenced hash is no longer on the specified branch.

Signed-off-by: Roy Franz <rfranz@tenstorrent.com>