iio: stx104: Add GPIO set_multiple callback function support

The Apex Embedded Systems STX104 series provides a digital output
register where 4 lines may be set at a time. This patch add support for
the set_multiple callback function, thus allowing multiple digital
output lines to be set more efficiently in groups.

Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>