[POWERPC] Add of_register_i2c_devices()

Scan the device tree for i2c devices, check their "compatible" property
against a hard-coded table, and, if found, register with i2c boardinfo.
This provides the infrastructure needed to find i2c devices in the
device tree and register them with the i2c subsystem.

This and the following commit let the linkstation work with the new i2c
API and thus fix a regression.

Signed-off-by: G. Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>