pinctrl: bcm2835: declare pin config as generic

Since commit 0de70495 ("pinctrl: bcm2835: Add support for
generic pinctrl binding") this driver is capable to use the generic
interface. So declare this accordingly.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>