staging: fieldbus: arcx-anybus: change custom -> mmio regmap

The arcx-anybus's registers are accessed via a memory-mapped
IO region. A regmap associated with this region is created
using custom reg_read() / reg_write() callbacks.

However, an abstraction which creates a memory-mapped IO
region backed regmap already exists: devm_regmap_init_mmio().

Replace the custom regmap with the existing kernel abstraction.
As a pleasant side-effect, sparse warnings now disappear.

Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>