drivers: rtc-rpi: add battery charge circuit control and readback

Parse devicetree for a charger voltage and apply it. If nonzero and a
valid voltage, the firmware will enable charging, otherwise the charger
circuit is disabled.

Add sysfs attributes to read back the supported charge voltage range,
the measured battery voltage, and the charger setpoint.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>