ARM: zynq: Replace 'io-standard' with 'power-source' property

Replace 'io-standard' property with 'power-source' property in all
zynq dts files to be in sync with Zynq Pinctrl driver.

Signed-off-by: Sai Krishna Potthuri <lakshmi.sai.krishna.potthuri@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>