cmake: dts: Add dt_comp_path() cmake function

Add a new cmake extension function:

  dt_comp_path(<var> COMPATIBLE <compatible> [INDEX <idx>])

to get a list of paths for the nodes with the given <compatible>. This
is useful when we have to cycle through several nodes with the same
compatible.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>