regulator: ab8500-ext: Register as a device in its own right

Some platforms don't support the AB8500 external regulators, so instead
of having a list of is_<platform>() calls prior to calling
ab8500_ext_regulator_init() from ab8500_regulator_probe(), we can only
register as a platform device on platforms which require them. It means
we also have more control over them when booting with Device Tree.

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