pinctrl: qcom: spmi-mpp: Transpose pinmux function

The "function" of the MPP driver was inherited from the GPIO driver, but the
differences between the two hardware blocks makes both the driver and the
device tree binding to be awkward.

Instead of overloading the "normal" function with various modes this patch
transposes the pinmux function to represent the three operating modes of the
MPP (digital, analog and current sink). The properties of pin pairing and DTEST
routing is moved to separate properties.

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