libplatsupport: workaround macro for clang

clang doesn't like the idea of using function pointers
in static_assert. we use c preprocessor magic to detect
if user is passing NULL or 0 as compat_list or init_func
for a driver_module

libplatsupport: Add missing header include

platsupport's io.h describes the ps_io_ops_t structure and was missing
from this header file.

libplatsupport: Use const for static strings

This is more compatible with existing interfaces.

libplatsupport: Add driver module definitions

These definitions are used to allow the user to register driver modules.
For now, these are only used in the context of CAmkES environments to
allow specific modules to be initialised all at once during the
initialisation phase of a component's lifetime.