regulator: ab8500: Don't register external regulators on AB8505

ExtSupply regulators are not included on AB8505 based platforms.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Tested-by: Marcus COOPER <marcus.xm.cooper@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>