mfd: Move ssbi driver into drivers/mfd

mfd: Move ssbi driver into drivers/mfd

There is no reason for ssbi to have its own top-level driver directory
when the only users of this interface are all MFD drivers. The only
mainline driver using it at the moment (PM8921) is marked broken and in
fact does not compile. I have verified that fixing the trivial build
breakage in pm8921 links in the new ssbi code just fine, but that
can be a separate patch.

Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Greg Kroah-Hartman <[email protected]>
Acked-by: Nicolas Pitre <[email protected]>
Acked-by: David Brown <[email protected]>
Signed-off-by: Samuel Ortiz <[email protected]>