scripts/dts: Make all_compats() return a set with all compats

Previously, all_compats() returned a dictionary mapping nodes to their
'compatible' values. This dictionary was then (only) used to generate a
set() of all 'compatible' strings.

Save a step and get rid of some code by having all_compats() return a
set() with all 'compatible' strings directly.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>