MAINTAINERS: Add entry for the Nitro Enclaves driver

Add entry in the MAINTAINERS file for the Nitro Enclaves files such as
the documentation, the header files, the driver itself and the user
space sample.

Changelog

v9 -> v10

* Update commit message to include the changelog before the SoB tag(s).

v8 -> v9

* Update the location of the documentation, as it has been moved to the
  "virt" directory.

v7 -> v8

* No changes.

v6 -> v7

* No changes.

v5 -> v6

* No changes.

v4 -> v5

* No changes.

v3 -> v4

* No changes.

v2 -> v3

* Update file entries to be in alphabetical order.

v1 -> v2

* No changes.

Reviewed-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
Link: https://lore.kernel.org/r/20200921121732.44291-19-andraprs@amazon.com


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