The driver has lots of places with chip-specific code what doesn't
necessarily facilitate maintenance.

Let's describe chip-specific differences in century bit handling
in struct chip_desc to improve this.
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>