bcm2835_sdhci: add some very basic support for rpi4

DMA is currently disabled while I work out why it's broken, but this is
enough for upstream U-Boot + rpi-firmware + our rpi3-psci-monitor to boot
with the right config.

The RPi 4 is still not in a good "supported" state, as we have no
USB/PCI-E/Ethernet drivers, but if air-gapped pies only able to operate over
cereal is your thing, here's your guy.

Submitted by: Robert Crowston (with modifications)