[RFC] dtc: add ability to make nodes conditional on them being referenced

Hi,

Heiko St?bner wrote:

From: Heiko Stuebner <redacted>

On i.MX, which carries a lot of pin-groups of which most are unused on
individual boards, they noticed that this plehora of nodes also results
in the runtime-lookup-performance also degrading [0].

A i.MX-specific solution defining the pingroups in the board files but
using macros to reference the pingroup-data was not well received

This patch is trying to solve this issue in a more general way, by
adding the ability to mark nodes as needing to be referenced somewhere
in the tree.

To mark a node a needing to be referenced it must be prefixed with
/delete-unreferenced/. This makes dtc check the nodes reference-status
when creating the flattened tree, dropping it if unreferenced.

For example, the i.MX6SL pingroup

	/delete-uneferenced/ pinctrl_ecspi1_1: ecspi1grp-1 {

                 ^^
s/unef/unref/
not really important here, but bad for copy/pasters


Lothar Wa?mann
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