dt-bindings: arm-ccn: Add bindings info for CCN-502 compatible string

Add CCN-502 to the list of supported devices by ARM CCN PMU driver.

Signed-off-by: Velibor Markovski <velibor.markovski@broadcom.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>