fpga: virtex2: added Kconfig option

Add an option to allow this driver to be selected with Kconfig. As noted
in the description, this driver should also work with many newer Xilinx
FPGA families as the programming methods are essentially the same.

Also added a missing FPGA_XILINX dependency to the similar Spartan 3
driver.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>