checkpatch: Add exceptions for "dsb" keyword usage

Currently checkpatch.pl throws error for any use of "dsb"
keyword.  Add exceptions when it is used in a variable or a
function name.  Expectations are when 'dsb' keyword is used in a
variable or a function name, it is prefixed with class [-_>*\.]
and/or suffixed with class [-_\.;].

Change-Id: I06cd21e42135334bb41cf31f13088b118fcf3ac9
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>