ARM: zynq: DT: Add zc702 pushbuttons to DT as gpio-keys

Adds the two MIO connected pushbuttons on the zc702 board to the devicetree as a
single multi-key device for us with the gpio-keys driver.

Signed-off-by: Ezra Savard <ezra.savard@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>