usb: Kconfig: Add Kconfigs entry USB_EHCI_ZYNQ

Add Kconfig entry config option for USB_EHCI_ZYNQ
and update the same to enable for all zynq boards
which supports USB

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>