pinctrl: bcm2835: add pull defines to dt bindings

Also delete (unused) private enum from driver.
The pull defines can be used instead if needed.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>