gpio: zynq: Setup chip->base based on alias ID

In past Xilinx gpio-zynq driver was setting up gpio chip->base as 0
which was chagned to autodetection when driver was upstreamed. Older
systems, which were using this old version, setup SW stack which expects
zynq gpio base as 0 and right now there is no way how to set this up.

The patch is adding an option to setup chip->base based on aliases which
is something what some other drivers are doing too.
It means when gpio0 alias is setup then chip->base is 0. When gpio alias
is not setup gpiochip_find_base() set it up properly which is current
behavior.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>