mfd ab8500-gpadc: Introduce new AB version detection

Add support for AB8505 and AB9540

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@stericsson.com>
Signed-off-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Reviewed-by: Rabin VINCENT <rabin.vincent@stericsson.com>