The main UART on the Xilinx ZC702 board is UART1, located at address
e0001000.  Add a Kconfig option to select this device as the low-level
debugging port.  This allows the really early boot printouts to reach
the USB serial adaptor on this board.

For consistency's sake, add a choice entry for UART0 even though it is
the the default if UART1 is not selected.
Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
Tested-by: Josh Cartwright <josh.cartwright@ni.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>