dt: add helpers for multi-cell adds

dt: add helpers for multi-cell adds

We have device tree helpers that allow us to create single cell (u32)
wide properties. However, when creating properties that contain an array of
cells, we need to jump through hoops, manually passing in an array with
converted endianness.

To ease the pain of this, create a generic macro helper that allows us
to pass the cells as arguments.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@petalogix.com>