Add a pair of convenience routines for doing simple "register" read/writes
on i2c devices, where the "register" can be any length.

Many (perhaps most) common i2c devices are organized as a collection of
(usually 1-byte-wide) registers, and are accessed by first writing a 1-byte
register index/offset number, then by reading or writing the data.
Generally there is an auto-increment feature so the when multiple bytes
are read or written, multiple contiguous registers are accessed.

Most existing slave device drivers allocate an array of iic_msg structures,
fill in all the transfer info, and invoke iicbus_transfer(). These new
functions commonize all that and reduce register access to a simple call
with a few arguments.