regulator: ab8500-ext: Add support for AB9540 regulators

Add the support for ab9540 external regulators.

Signed-off-by: Michel JAOUEN <michel.jaouen@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>