Fix u-boot for rock-5b (and update u-boot for rock-5a)

[kwankiu]

The current BredOS image does not boot on Rock 5B.
Manually flashing the u-boot solved it.
I think that updating the u-boot will solve this issue.
And this radxa rock 5b u-boot seems works well with USB PD.

# extract radxa u-boot
# replace u-boot-rknext_2017.09-10-52701b7_arm64.deb with the latest version number available
curl -L https://github.com/radxa-repo/bullseye/raw/gh-pages/.pool/u-boot-rknext_2017.09-10-52701b7_arm64.deb -o uboot.deb
ar xv uboot.deb
tar -xf data.tar.xz
# u-boot is located at usr/lib/u-boot/rock-5a or usr/lib/u-boot/rock-5b