drivers/sensor: Add basic Silabs Si7210 support

This adds basic support for the Silabs Si7210 hall effect magnetic
position and temperature sensor. It is able to get magnetic field and
temperature in the default scale of the sensor (depending on the
variant). It also supports going into sleep mode without measurements
through the device power management infrastructure.

It is most notably missing support for scale change, measurement
averaging and filtering, and alert pin configuration (threshold,
hysteris, tamper).

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>