video: bochs: Remove the x86 dependency

Now that the driver is legacy free, remove the x86 dependency so
that it can be used on non-x86 architectures.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>  # qemu-x86_64