iio: accel: bma180: Prepare for different reset values

Some variants of the bma180 (eg bma023) have different reset
values.  In preparation for adding support for them, factor
out the reset value into the chip specific data.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>