powerpc/boot: Only build uartlite if XILINX_VIRTEX=y

The serial code in uartlite.c only matches if we find one of two
Xilinx (xlnx) nodes in the device tree, there's no need to build or
link the code on other platforms.

As far as I can tell CONFIG_XILINX_VIRTEX is the appropriate symbol to
use to conditionally compile the code.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>