configs/beaglebone: bump kernel and U-Boot versions

- Bump kernel to version 5.10.30-ti-r3.
- Bump U-Boot to version 2021.04.

Add mdev to the beaglebone_defconfig. This gives us automatic USB support,
as the omap2plus kernel has most drivers as modules.

While at it, enable VFPv3 with 32 registers (instead of 16) and add a few
comments to the defconfig.

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Cc: Lothar Felten <lothar.felten@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>