arm: zynq: Drop #address-cells and #size-cells from gpio-keys

dtc is showing some warnings and this change was also done in
the Linux kernel as "Input: gpio-keys - clean up device tree binding
example"
with this fragment in commit message
"Drop #address-cells and #size-cells, which are not required by the
gpio-keys binding documentation, as button sub-nodes are not devices."

Signed-off-by: Michal Simek <michal.simek@xilinx.com>