vf_i2c: split up and add ACPI attachments in addition to FDT

Move the code from the arm specific to the iicbus controller directory.
Split up between general logic and bus attachment code.
Add support for ACPI attachment in addition to FDT.

MFC after: 7 days
Tested by: bz (LS1088a FDT), Pierre-Luc Drouin (Honeycomb, ACPI)
Based on: D24917 by Val Packett (initial early version)
Differential Revision: https://reviews.freebsd.org/D44020