gpio/nomadik: add function to read GPIO pull down status

Signed-off-by: Rickard Andersson <rickard.andersson@stericsson.com>
Reviewed-by: Martin Persson <martin.persson@stericsson.com>
[Split off from larger patch]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>