vf_i2c: update I2C controller logic

Update the I2C controller logic to be more consistent with the
newer version of the controller reference manual.
This makes it work better on modern LS/LX platforms and avoids
unnecessary delays. Also fixes a lock leak.

MFC after: 7 days
Tested by: bz (LS1088a FDT), Pierre-Luc Drouin (Honeycomb, ACPI)
Differential Revision: https://reviews.freebsd.org/D44021

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