arm: zynq: zybo z7: fix MIO init issue

The board has two push button connected to MIO pins
50 and 51, which have a pull-down resistor and are
connected to 1.8V when pressed.

These two pins are wrongly initialized with internal
pull-up enabled so they are reported as 1 all the time
with no change when pressed.

Disable the internal pull-up to fix the issue.

Signed-off-by: Milan Obuch <u-boot@dino.sk>
Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>