gpio: pcie-idio-24: Implement get_multiple/set_multiple callbacks

The ACCES I/O PCIe-IDIO-24 series of devices provides 24
optically-isolated digital I/O accessed via six 8-bit ports. Since eight
input lines are acquired on a single port input read -- and similarly
eight output lines are set on a single port output write -- the
PCIe-IDIO-24 GPIO driver may improve multiple I/O reads/writes by
utilizing a get_multiple/set_multiple callbacks. This patch implements
the idio_24_gpio_get_multiple function which serves as the respective
get_multiple callback, and implements the idio_24_gpio_set_multiple
function which serves as the respective set_multiple callback.

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