uio: uio_dfl: add userspace i/o driver for DFL bus

uio: uio_dfl: add userspace i/o driver for DFL bus

This patch supports the DFL drivers be written in userspace. This is
realized by exposing the userspace I/O device interfaces.

The driver now only binds the ether group feature, which has no irq. So
the irq support is not implemented yet.

Reviewed-by: Tom Rix <trix@redhat.com>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/1615168776-8553-2-git-send-email-yilun.xu@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>