shields: Settle on nodelabels naming scheme

[erwango]

When using shields with boards, it may happen that shields and board both describe use the same nodelabel (to describe the same device: sensor, display, ..), leading to a conflict at device tree compilation.
For instance, same node label 'lsm6dsl' is used for both board and shield description:

zephyr/boards/shields/x_nucleo_iks01a2/x_nucleo_iks01a2.overlay

Lines 28 to 30 in eafc4f8

	lsm6dsl: lsm6dsl@6b {
	compatible = "st,lsm6dsl";
	reg = <0x6b>;

zephyr/boards/arm/b_l4s5i_iot01a/b_l4s5i_iot01a.dts

Lines 113 to 115 in eafc4f8

	lsm6dsl: lsm6dsl@6a {
	compatible = "st,lsm6dsl";
	reg = <0x6a>;

This was discussed initially in #48780.

Conclusion of the discussion was to use, in shields device tree description, the following device nodelabel naming scheme:
<device>_<address>

Then #50034 was merged using a different approach:
<device>_<shield_name>
Reason is that <device>_<address> will give conflicts (in dtc) when using multiple shields using devices at same address but on different busses.
Note that this proposal was initially rejected in #48780 because not concise enough.

Aim of this issue is to settle on agreement that can be applied in all cases, document it and update in-tree shields accordingly

[erwango]

@gmarull @MaureenHelm, fine with using <device>_<shield_name> if required to solve the same address/ multiple busses issue ?

[gmarull]

I think it makes sense

[erwango]

@MaureenHelm ?

[MaureenHelm]

Agree this makes sense

[erwango]

Agree this makes sense

ok, thanks. I'll update