regulator: axp20x: Add module alias

This allows the module to be autoloaded.

Together with 07949bf9

 ("cpufreq: dt: allow driver to boot
automatically") this is sufficient to allow a modular kernel (such
as Debian's) to enable cpufreq on a Cubietruck.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>