patman: invoke the checkpatch.pl script with '--u-boot' and '--strict'

This resolves 10 out of 11 test failures seen when running './patman
test' from the 'tools/patman' subdirectory. This was caused by the
.checkpatch.conf configuration file at the root of the project not
being picked up. Make the test suite of patman independent from it by
always invoking the checkpatch.pl script with the minimally required
arguments for the test suite to pass.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>