checkpatch: fix left brace warning

using checkpatch with Perl 5.22.0 generates a warning,
this is fixed in linux through commit:

commit 4e5d56bdf892e18832a6540b63ebf709966bce2a
Author: Eddie Kovsky <ewk@edkovsky.org>
Date:   Wed Sep 9 15:37:52 2015 -0700

checkpatch: fix left brace warning

Using checkpatch.pl with Perl 5.22.0 generates the following warning:

Unescaped left brace in regex is deprecated, passed through in regex;

This patch fixes the warnings by escaping occurrences of the left brace
inside the regular expression.

Adapt it for U-Boot.

Signed-off-by: Heiko Schocher <hs@denx.de>