rpi-firmware: update to a recent commit

The purpose of this update being primarily for RPi4 support to ease the
burden of anyone wanting to try -CURRENT and assist with driver -- note that
we currently don't support ethernet/usb/pci on the RPi4.

Some cleanup has also been done to not duplicate variables that the ports
framework already provides (${WRKSRC_${tag}}) and simplify.

This update does include a rename of some dtb that will require a
corresponding change in base to ^/release, which will follow this commit.

Approved by: manu (maintainer (uboot), ports)