pinctrl: bcm2835: Implement bcm2835_pinconf_get

Even the driver already has implemented pin_dbg_show, it could
be helpful to implement pin_conf_get for a more generic behavior.
Contrary to the BCM2711, the BCM2835 SOC doesn't allow to read
the bias config, so the implementation is limited to the basics.

Keep ENOTSUPP here, because it's only used internally.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Message-ID: <20240307070113.4888-2-wahrenst@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>