iio: stx104: Add support for GPIO names

This patch sets the gpio_chip names option with an array of GPIO line
names that match the manual documentation for the Apex Embedded Systems
STX104. This should make it easier for users to identify which GPIO line
corresponds to a respective GPIO pin on the device.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>