fsi: occ: Support probing the hwmon child device from dts node

fsi: occ: Support probing the hwmon child device from dts node

There is now a need for reading devicetree properties in the OCC
hwmon driver, which isn't current supported as the FSI driver just
instantiates a basic platform device. Add support for this use case
by checking for an "occ-hwmon" node and if present, creating an
OF device from it.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20220809200701.218059-3-eajames@linux.ibm.com
Signed-off-by: Joel Stanley <joel@jms.id.au>