fix: mpfs-beaglev-fire

This is a follow up to !102 (merged).

Fixes the SD Card slot being permanently disabled.

Also pins eMMC to mmc0, preventing sd-card from "stealing" it and failing to boot, since the kernel cmdline expects mmc0 to be the boot source.

This closes issues #57 (closed) and beaglev-fire/BeagleV-Fire-ubuntu#10 (closed).

@RobertCNelson could I ask that you give it a whirl before merging, just to make sure it actually works this time... Thanks!