gpio: Added zynq specific check for special pins on bank zero

This patch adds zynq specific check for bank 0 pins 7 and 8
are special and cannot be used as inputs

Signed-off-by: Nava kishore Manne <navam@xilinx.com>
Reported-by: Jonas Karlsson <Jonas.d.karlsson@gmail.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>