checkpatch: add Suggested-by as a standard signature

checkpatch: add Suggested-by as a standard signature

As people started using Suggested-by as standard signature, adding
"Suggested-by" to the standard signature so that checkpatch won't
generate warning when Suggested-by is used in patch signature

Signed-off-by: Mugunthan V N <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Joe Perches <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>