serial: rsci: Add set_rtrg() callback

serial: rsci: Add set_rtrg() callback

[ Upstream commit b346e5d7dbf6696176417923c49838a1beb1d785 ]

The rtrg variable is populated in sci_init_single() for RZ/T2H. Add
set_rtrg() callback for setting the rtrg value.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/20251129164325.209213-4-biju.das.jz@bp.renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>