pinctrl: mvebu: identify generic controls by name

We treat unnamed controls as generic mvebu mpp register controls but
we identify them by not being special controls. Flip the logic and
use the name pointer as identification instead. While at it, add some
comments explaining the not so obvious treatment.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>