dts: edtlib/gen_defines: Fix API design re. dtc flags

It's better to allow per-instance EDT configuration than to set a global
variable on the edtlib module. Enable/disable the warning for reg/unit
address mismatches via a flag to EDT.__init__(), instead of via a global
variable. That makes it consistent too.

Another option would be to pass the 'dtc' flags to EDT.__init__(), but
it makes the interface a bit ugly. Maybe if it needs to emulate lots of
other flags later.

Clarify that edtlib itself isn't meant to have any state in the comment
at the top of the module.

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