boards: arduino_nicla_sense_me: pinctrl fixes

Two pinctrl node labels for this board are referring to nodes that are
already defined higher in the file with different pinmux property
values.

This looks like a clear copy/paste error. Fix it by renaming the nodes
to create two distinct nodes in each case instead of overriding the
previous node definition. This will also make the file compile in dtc,
which regards duplicate node definitions as errors, and will be a
required fix for when Zephyr's dtlib does the same.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>