usertools: add option to unbind all devices

usertools: add option to unbind all devices

-u accepts "dpdk" argument to unbind all devices bound to a DPDK driver.

Usage:
usertools/dpdk-devbind.py -u dpdk

Example:
$ usertools/dpdk-devbind.py -s

Network devices using DPDK-compatible driver
============================================
0000:08:00.1 '...' drv=igb_uio unused=
0000:81:00.0 '...' drv=igb_uio unused=
0000:88:00.0 '...' drv=igb_uio unused=
0000:88:00.1 '...' drv=igb_uio unused=
...

$ usertools/dpdk-devbind.py -u dpdk
$ usertools/dpdk-devbind.py -s

Network devices using DPDK-compatible driver
============================================
<none>
....

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>