drm/i915/hwmon: Add HWMON current voltage support

Use i915 HWMON subsystem to display current input voltage.

v2:
  - Updated date and kernel version in feature description
  - Fixed review comments (Ashutosh)
v3: Use macro HWMON_CHANNEL_INFO to define hwmon channel (Guenter)
v4:
  - Fixed review comments (Ashutosh)
  - Use hwm_ prefix for static functions (Ashutosh)
v5: Added unit of voltage as millivolts (Ashutosh)
v6: KernelVersion: 6.2, Date: February 2023 in doc (Tvrtko)
v7: Change contact to intel-gfx (Rodrigo)
    GEN12_RPSTAT1 is available for all Gen12+ (Andi)
    Added Documentation/ABI/testing/sysfs-driver-intel-i915-hwmon
    to MAINTAINERS

Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@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>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013154526.2105579-3-ashutosh.dixit@intel.com