mailbox: bcm-flexrm-mailbox: Use bitmap instead of IDA

Currently, we are using IDA library for managing IDs
on a FlexRM ring. The IDA library dynamically allocates
memory for underlying data structures which can cause
potential locking issue when allocating/free IDs from
flexrm_new_request() and flexrm_process_completions().

To tackle this, we replace use of IDA with bitmap for
each FlexRM ring and also protect the bitmap with FlexRM
ring lock.

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