mfd: Add sysfs attributes for Kontron PLD firmware revision

This patch adds attributes to the Kontron PLD driver to allow
applications to retrieve firmware information.
Additionally the format has been changed to conform with the
representation in other Kontron software.

Signed-off-by: Michael Brunner <michael.brunner@kontron.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>