coreboot: Add coreboot firmware table driver

Nexus-attached driver that discovers and parses coreboot's LBIO tables
from physical memory. Exposes firmware metadata (version, build info,
mainboard, s

coreboot: Add coreboot firmware table driver

Nexus-attached driver that discovers and parses coreboot's LBIO tables
from physical memory. Exposes firmware metadata (version, build info,
mainboard, serial config, TSC frequency, CBMEM entries) via sysctl
hw.coreboot.*, the firmware console ring buffer via /dev/coreboot_console,
and structured CBMEM entry access via /dev/cbmem ioctl interface.

Tested on:
- Qotom Q535G6 (Kabylake)
- Intel NUC D54250WYK (Haswell)
- Intel NUC D33217GKE (Ivy Bridge)
- Dell 3100 2-in-1 (Gabbiter)
- Dell 3100 (Fleex)
- Lenovo IdeaPad 320s
- Lenovo ThinkPad T480
- HP Chromebook 11 G4
- HP Chromebook 11 G5
- HP Chromebook 11 G6 EE
- HP Chromebook 14 G4
- HP Chromebook 14 G5
- HP Chromebook x360 11 G1 EE
- HP Chromebook x360 11 G2 EE
- HP Chromebook x360 14 G1
- Acer C720
- Acer Chromebook 11
- Lenovo N22

Reviewed by: ngie, kib, adrian
Differential Revision: https://reviews.freebsd.org/D55649

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