staging: dpaa2-switch: move the driver out of staging

Now that the dpaa2-switch driver has basic I/O capabilities on the
switch port net_devices and multiple bridging domains are supported,
move the driver out of staging.

The dpaa2-switch driver is placed right next to the dpaa2-eth driver
since, in the near future, they will be sharing most of the data path.
I didn't implement code reuse in this patch series because I wanted to
keep it as small as possible.

Also, the README is removed from staging with the intention to add
proper rst documentation afterwards to actually match was is supported
by the driver.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>