xtensa: virt: add defconfig and DTS

Add defconfig and DTS for a virt board. Defconfig enables PCIe host and
a number of virtio devices. DTS routes legacy PCI IRQs to the first four
level-triggered external IRQ lines. CPU core with edge-triggered IRQs
among the first four may need a custom DTS to work correctly.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>