mailbox: bcm-flexrm-mailbox: Use common GPL comment header

This patch makes the comment header of Broadcom FlexRM driver
similar to the GPL comment header used across Broadcom driver
sources.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>