mailbox: bcm-flexrm-mailbox: Add debugfs support

This patch adds debugfs support to Broadcom FlexRM driver
so that we can see FlexRM ring state when any issue happens.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>