mailbox: remove superfluous internal header

mailbox: remove superfluous internal header

Quite some controller drivers use the defines from the internal header
already. This prevents controller drivers outside the mailbox directory.
Move the defines to the public controller header to allow this again as
the defines are not strictly internal anyhow.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Sudeep Holla <sudeep.holla@kernel.org>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>