bcm2835-virtgpio: Update for Linux 6.6

The gpio subsystem is happier if the gpiochip is given a parent, and
if it doesn't have a fixed base gpio number. While we're in here,
use the fact that the firmware node is the parent to locate it,
and use the devm_ version of rpi_firmware_get.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>