configs/rpi: fix defconfigs after upstream rebased

configs/rpi: fix defconfigs after upstream rebased

Branches in the Rappberry Pi linux repository are often rebased, which
means that commits that are not reachable from a reference (tag,branch)
will eventually get garbage-collected.

This is probably what hapenned with the commit we are curently
referencing in our defconfig files.

Swith to using the current HEAD of the rpi-5.10.y brnch, in lieue of the
previous one.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>