Input: add a driver to support Hyper-V synthetic keyboard

Add a new driver to support synthetic keyboard. On the next generation
Hyper-V guest firmware, many legacy devices will not be emulated and this
driver will be required.

I would like to thank Vojtech Pavlik <vojtech@suse.cz> for helping me with the
details of the AT keyboard driver. I would also like to thank
Dan Carpenter <dan.carpenter@oracle.com> and
Dmitry Torokhov <dmitry.torokhov@gmail.com> for their detailed review of this
driver.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>