linuxkpi: Add i2c support

Add i2c support to linuxkpi. This is needed by drm-kmod.
For every i2c_adapter added by i2c_add_adapter we add a child to the
device named "lkpi_iic". This child handle the conversion between
Linux i2c_msgs to FreeBSD iic_msgs.
For every i2c_adapter added by i2c_bit_add_bus we add a child to the
device named "lkpi_iicbb". This child handle the conversion between
Linux i2c_msgs to FreeBSD iic_msgs.
With the help of iic(4), this expose the i2c controller to userspace
allowing a user to query DDC information from a monitor.
e.g.: i2c -f /dev/iic0 -a 0x28 -c 128 -d r
will query the standard EDID from the monitor if plugged.

The bitbang part (lkpi_iicbb) isn't tested at all for now as I don't have
compatible hardware (all my hardware have native i2c controller).

Tested on: Intel (SandyBridge, Skylake, ApolloLake)
Tested on: AMD (Picasso, Polaris (amd64 and arm64))

MFC after: 1 month
Reviewed by: hselasky
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33053

(cherry picked from commit 1961a14a47437595fb7fcdc20e327440e3eb51e2)