hwmon (occ): Add command transport method for P8 and P9

For the P8 OCC, add the procedure to send a command to the OCC over I2C
bus. This involves writing the OCC command registers with serial
communication operations (SCOMs) interpreted by the I2C slave. For the
P9 OCC, add a procedure to use the OCC in-kernel API to send a command
to the OCC through the SBE.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>