loader.efi: Parse SPCR table entry in ACPI tables

If there's a SPCR, then use it to create and pass the right values to
the uart. We pass xo=0 in to calcuate the xo from the baud rate. We try
to be smart about what we set. We either set io or mm or pv/pd. Old
kernels will still work, despite pb/pd not being supported, because
we'll fall back to the SPCR parsing in the kernel.

We don't support Rev3 or Rev4 SPCR yet. It's too new to be in real
hardware yet.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D47085