gpio: 104-dio-48e: Implement get_multiple callback

The ACCES I/O 104-DIO-48E series of devices contain two Programmable
Peripheral Interface (PPI) chips of type 82C55, which each feature three
8-bit ports of I/O. Since eight input lines are acquired on a single
port input read, the 104-DIO-48E GPIO driver may improve multiple input
reads by utilizing a get_multiple callback. This patch implements the
dio48e_gpio_get_multiple function which serves as the respective
get_multiple callback.

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