Add TCA6416 GPIO expander support.

Add basic TCA6416 GPIO expander support over I2C bus. The driver handles
enabling and disabling pins, setting pin mode to IN and OUT and
toggling the pins. External interrupts are not supported.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: manu, mmel
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D24363