[dev][acpi] use uint8_t to cache last seen bbn

Previously the driver cast a int pointer to a uint8_t pointer to
pass to a function that returns the BBN. Because the default value
of the param was -1, the value is incorrect, causing a problem
where the NHLT metadata was published incorrectly.

ZX-2610 #done

Test: booted eve and verified the audio devices are published
Change-Id: I391d1b8c6168225b5b626102a5fb882e761b0c54

[bus][acpi] Pull pci/pciroot code out into their own file

This commit mostly moves some code from bus-acpi to pciroot
and makes small updates to C++ify it along the way.

Test: built / tested on x86, system still boots.

Change-Id: I25f3e700b62f4ed28408d9a75ddc1c3a89ad755c