ARM: dts: qcom: pm8226: Add node for the MPP

The PM8226 provides 8 Multi-Purpose Pins (or MPPs for short).
Add a node to support them.

Signed-off-by: Rayyan Ansari <rayyan@ansari.sh>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220124171538.18088-3-rayyan@ansari.sh