Documentation: fpga: dfl: Add description for DFL UIO support

This patch adds description for UIO support for dfl devices on DFL
bus.

Reviewed-by: Tom Rix <trix@redhat.com>
Reviewed-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/1615168776-8553-3-git-send-email-yilun.xu@intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>