Move board related device tree files where the board is defined

[zephyrbot]

Reported by Anas Nashif:

right now we put all device tree files for all boards into dts//, this means that when adding a board we need to manage it in 2 different places. We should look into maintaining board dts files in the board directory, optionally move the common files for an SoC and architectures to where the Soc is maintained, under arch//soc/...

This would enable us to maintain everything related to a board in one place and would make it easy for us to handle boards as drop-ins, i.e. maintain them out of tree as git submodules or through simple references.

(Imported from Jira ZEP-2148)

[galak]

This should be address with PR #4843

[galak]

Fixed by #4843