get_maintainer: fix perl 5.22/5.24 deprecated/incompatible "\C" use

from linux commit ce8155f7a3d5:

Perl 5.22 emits a deprecated message when "\C" is used in a regex.  Perl
5.24 will disallow it altogether.

Fix it by using [A-Z] instead of \C.

>From linux adapted to U-Boot by:
Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>