drm/i915/hwmon: Add HWMON infrastructure

The i915 HWMON module will be used to expose voltage, power and energy
values for dGfx. Here we set up i915 hwmon infrastructure including i915
hwmon registration, basic data structures and functions.

v2:
  - Create HWMON infra patch (Ashutosh)
  - Fixed review comments (Jani)
  - Remove "select HWMON" from i915/Kconfig (Jani)
v3: Use hwm_ prefix for static functions (Ashutosh)
v4: s/#ifdef CONFIG_HWMON/#if IS_REACHABLE(CONFIG_HWMON)/ since the former
    doesn't work if hwmon is compiled as a module (Guenter)
v5: Fixed review comments (Jani)
v6: s/kzalloc/devm_kzalloc/ (Andi)
v7: s/hwmon_device_register_with_info/
      devm_hwmon_device_register_with_info/ (Ashutosh)

Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dale B Stimson <dale.b.stimson@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013154526.2105579-2-ashutosh.dixit@intel.com