staging:iio:hmc5843: Added regmap support

This patch changes hmc5843.c to use regmap. This provides transparent caching
to the code as well as abstraction necessary to add support for SPI-based
hmc5983.

Signed-off-by: Josef Gajdusek <atx@atx.name>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>