checkpatch: Recognized volatile as part of a declaration.

Adjust the checkpatch script to recognize the volatile keyword as
forming part of a declaration.  This prevents inappropriate checkpatch
gripes for patches such as:

https://gerrit.zephyrproject.org/r/#/c/10345/



Change-Id: Ia29a9002bb9ef15f747d29231d938ae12790724d
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>