openrisc: Add virt defconfig

I have been developing a new qemu virt platform to help with more
efficient toolchain and kernel testing [1].

This patch adds the defconfig which is needed to support booting
linux on the platform.

[1] https://lore.kernel.org/qemu-devel/YpwNtowUTxRbh2Uq@antec/T/#m6db180b0d682785fb320e4a05345c12a063e0c47



Signed-off-by: Stafford Horne <shorne@gmail.com>