pinctrl: bcm2835: Return pins to inputs when freed

When dynamically unloading overlays, it is important that freed pins are
restored to being inputs to prevent functions from being enabled in
multiple places at once.

Cc: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>