dts: edtlib: Turn Node.instance_no into EDT.compat2enabled

Add an EDT.compat2enabled attribute that maps compatibles to enabled
devicetree nodes that implement them. For example,
EDT.compat2enabled["foo"] is a list of all enabled nodes with "foo" in
the 'compatible' property.

The old Node.instance_no functionality can be implemented in terms of
EDT.compat2enabled, so remove Node.instance_no. EDT.compat2enabled is
more flexible and easier to understand.

Simplify main() in gen_defines.py by using EDT.compat2enabled to
generate the DT_COMPAT_<compatible> existence macros. The behavior is
slightly different now, as DT_COMPAT_<compatible> is generated for
enabled nodes that don't have a binding as well, but that might be an
improvement overall. It probably doesn't hurt at least.

EDT.compat2enabled simplifies the implementation of the new
$(dt_compat_get_str) preprocessor function in
https://github.com/zephyrproject-rtos/zephyr/pull/21560

. That was the
original motivation.

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