MAINTAINERS: Add thunderbolt.rst to the Thunderbolt driver entry

Make sure Thunderbolt maintainers get to see patches that touch
documentation of the Thunderbolt driver as well.

Signed-off-by: Mika Westerberg <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

MAINTAINERS

···
 M:	Yehezkel Bernat <[e-mail]>
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/westeri/thunderbolt.git
 S:	Maintained
+F:	Documentation/admin-guide/thunderbolt.rst
 F:	drivers/thunderbolt/
 F:	include/linux/thunderbolt.h