tools/check_copyright: Allow multiline copyright

The current regex does not allow the copyright header to be carried in
the next line.
This is a limitation when updating the copyright by adding the current
year.

Update the regex to allow the copyright to be split into multilines.

Change-Id: Ibe51dfa0eb8bcef771c935847f0a9e651bf59298
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>