pinctrl: qcom: spmi-mpp: Add support for setting analog output level

When the MPP is configured for analog output the output level is selected by
the AOUT_CTL register, this patch makes it possible to control this.

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