hwmon: (ibmpowernv) Add attributes to enable/disable sensor groups

OPAL firmware provides the facility for some groups of sensors to be
enabled/disabled at runtime to give the user the option of using the
system resources for collecting these sensors or not.

For example, on POWER9 systems, the On Chip Controller (OCC) gathers
various system and chip level sensors and maintains their values in
main memory.

This patch provides support for enabling/disabling the sensor groups
like power, temperature, current and voltage.

Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
[stewart@linux.vnet.ibm.com: Commit message]
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>