drivers: pinctrl: silabs: Add support for analog bus allocation

The GPIO peripheral on Silabs Series 2 devices is responsible for
allocating analog buses to analog peripherals. Enable support for
this in the pinctrl driver. Since these bus allocations are not
digital pins, introduce a new property silabs,analog-bus for this
purpose.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>