scripts: Dynamically add driver subsystems to subsystems list

This change extends the parse_syscalls.py script to scan for a
__subsystem sentinal added to driver api declarations. It thens
generates a list that is passed into gen_kobject_list.py to extend
the subsystems list. This allows subsystems to be declared in the
code instead of a separate python list and provides a mechanism for
defining out-of-tree subsystems.

Signed-off-by: Corey Wharton <coreyw7@fb.com>