hwmon: (raspberrypi) Add voltage input support

hwmon: (raspberrypi) Add voltage input support

Extend the raspberrypi-hwmon driver to expose firmware-provided
voltage measurements through the hwmon subsystem.

The driver now exports the following voltage inputs:

  - in0_input (core)
  - in1_input (sdram_c)
  - in2_input (sdram_i)
  - in3_input (sdram_p)

Voltage values returned by firmware are converted from microvolts
to millivolts as expected by the hwmon subsystem.

Update the documentation related to it.

The existing undervoltage sticky alarm handling is preserved and
associated with the first voltage channel.

Tested in -
- Raspberry Pi 3b+ (Linux raspberrypi 6.12.75+rpt-rpi-v8 #1 SMP PREEMPT
  Debian 1:6.12.75-1+rpt1 (2026-03-11) aarch64 GNU/Linux)

Signed-off-by: Shubham Chakraborty <chakrabortyshubham66@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20260517080445.103962-3-chakrabortyshubham66@gmail.com
[groeck: Added missing empty line after declaration]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>