pinctrl: qcom: spmi-mpp: Add compatible for pmi8994

The PMI8994 has 4 multi-purpose-pins, add a compatible for this hardware
block to the MPP driver.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210429003751.224232-1-bjorn.andersson@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>