scripts/dts: globals: parse gpio-map properties for connector nodes

gpio-map is a property of "nexus node", defined in dts v0.3.
It allows to describe a pin connector so it can be referenced
through phandles and hence used in expansion device nodes like a
shield header (typically implemented through overlays).

This change implements gpio controller resolution through these maps.
Few assumptions were taken in order to simplify the implementation.
These assumptions bring some limitations to the use of gpio-map
but my understanding is that this should still allow to cover most
use cases.

Assumptions:
-gpio-size is the same for all gpio-controllers referenced in a map
-optional properties gpio-map-mask and gpio-map-pass-thru are
supposed to be omitted
The understanding of this last assumption is that flags provided in
the expansion device node will overwrite the connector flags.

In a latter stage, when need happen, these limitations can be
revisited to unlock fully fledged gpio-map usage.

Fixes #15637

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>