sff: Add SFP driver (fdt-based draft)

This basic version of the driver obtains properties of the "sff,sfp"
compatible devices and implements a simple interface to provide an I2C
bus device for the rest of the drivers (e.g. to implement SIOCGI2C).

Both of the interface and driver are subjects for a further
generalization to be used in case of non-FDT and non-arm64 platforms.

Reviewed by: bz, manu
Approved by: bz (mentor)
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D41440