dts: arm: microchip: mec127x: add /omit-if-no-ref/ to pinctrl nodes

All DT nodes end up being part of the generated 'devicetree_unfixed.h'
header, wether they are referenced or not. The number of entries in that
file can grow quickly when using pre-generated pinctrl nodes.
Considering <devicetree.h> (file uncluding devicetree_unfixed.h) is used
in lots of places nowaday, not using /omit-if-no-ref/ can lead to
increased build times.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>