powerpc/dts: add device tree entry for W83793 on T4240RDB

The T4240RDB contains a W83793 hardware monitoring chip. Add a device
tree entry to make the driver attach to it, as the i2c-mpc bus driver
dropped support for class-based instantiation of devices a long time
ago.

Signed-off-by: Florian Larysch <fl@n621.de>
Signed-off-by: Scott Wood <oss@buserror.net>