mailbox: ti-msgmgr: Remove use of of_match_ptr() helper

When OF support is disabled the of_device_id struct match table can be
conditionally compiled out, this helper allows the assignment to also be
turned into a NULL conditionally. When the of_device_id struct is not
conditionally defined based on OF then the table will be unused causing a
warning. The two options are to either set the table as _maybe_unused, or
to just remove this helper since the table will always be defined.
Do the latter here.

Signed-off-by: Andrew Davis <afd@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>