iio: accel: bma180: Basic regulator support

This brings up the VDD and VDDIO regulators using the
regulator framework. Platforms that do not use regulators
will provide stubs or dummy regulators.

Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>