regulator: ab8500: Ensure AB8500 external registers are probed first

This patch changes the order in which the AB8500 regulator drivers are
registered and subsequently probed. It saves a lot of -EPROBE_DEFEER
nonsense and bootlog noise, as some AB8500 core regulators depend on
the external ones for supply voltage.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>