gpio: pl061: Support building as module

gpio: pl061: Support building as module

Enable building the PL061 GPIO driver as a module.

This does change the initcall level when built-in. This shouldn't be a
problem as any user should support deferred probe by now. A scan of DT
based platforms at least didn't reveal any users that would be a
problem.

Cc: Linus Walleij <[email protected]>
Cc: Bartosz Golaszewski <[email protected]>
Cc: [email protected]
Signed-off-by: Rob Herring <[email protected]>
Signed-off-by: Bartosz Golaszewski <[email protected]>