drivers: syscon: do not assume that a function isn't supported

When a function pointer is `NULL`, it could be that the underlying
hardware doesn't support it, or it isn't implemented.

If a function isn't supported by the hardware, its driver should
return `-ENOTSUP`, the API layer should return `-ENOSYS` when a
function isn't implemented.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>