doc: porting: shields: Provide guidelines for nodelabel

In order to avoid device definitions conflicts when compiling shields
description with boards embedding similar devices, nodelabels of devices
in shield's devicetree file should differ from the nodelabels used in
board's devicetree file.
The form to be used was discussed in #50040 and agreed to be as
<device>_<shield_name>.

Update shields documentation to make it an explicit rule.

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