gpio: brcmstb: Use dynamic GPIO base numbers

Forcing a gpiochip to have a fixed base number now leads to a warning
message. Remove the need to do so by using the offset value of the
gpiochip.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240423184605.2094376-1-opendmb@gmail.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>