powerpc/4xx: Enable SERIAL_OF support by default for Virtex platforms

Virtex FPGA designs have two serial port logic cores to choose from; the
simple uartlite, and the full featured uart16550.  Both cores are in
common use so the defconfig should support both of them.  Currently
only console on uartlite is supported in the defconfig.  This patch adds
console support for the 16550 core.

The Virtex reference designs do not work without this patch.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>