[POWERPC] i2c: OF helpers for the i2c API

This implements various helpers to support OF bindings for the i2c
API.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>