[TC] mips: pmag-ba-fb: Convert to the driver model

This is a set of changes to convert the driver to the driver model.  As a
side-effect the driver now supports building as a module.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: James Simmons <jsimmons@infradead.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>