MFD: ucb1x00-core: add handling for ucb1x00 reset

Provide a way to handle the software controlled ucb1x00 reset signal
from the ucb1x00-core driver without having to code platform specifics
into these drivers.

Acked-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>