pinctrl: bcm2835: Implement get_direction callback

Implement gpio_chip's get_direction() callback, that lets other
drivers get particular GPIOs direction using gpiod_get_direction().

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