regulator: ab8500-ext: Add support for AB8505/AB9540

The external regulator driver checks for old AB8500 HW
to apply a fix. This patch adds a check to see that
it is a AB8500 (not AB8505/AB9540).

Signed-off-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>