scripts: gen_defines: allow multiple --vendor-prefixes

Allowing multiple such files will let higher layers take inputs from
multiple DTS_ROOT directories. This in turn will allow out of tree
bindings to manage their own sets of prefixes without patching
upstream Zephyr's file or doing other similar hacks.

Parse each file into a dict, and merge those dicts into a single
dictionary for the EDT constructor.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>