staging: board: Initial board staging support

Add staging board base support to allow continuous upstream
in-tree development and integration of platform devices.

Helps developers integrate devices as platform devices for
device drivers that only provide platform device bindings.
This in turn allows for incremental development of both
hardware feature support and DT binding work in parallel.

Two separate pieces of board staging functionality is
provided to ease per-board staging board support:
 - The board_staging() macro allows easy per-board callbacks
 - The board_staging_dt_node_available() provides DT node checking

Tested on the KZM9D board with the emxx_udc staging driver.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>