pinctrl: qcom: Add BUS_HOLD pin bias

This patch adds the BUS_HOLD (Keeper) bias option for pins.

Signed-off-by: Andy Gross <agross@codeaurora.org>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>