pwm: rz-mtu3: Fix build warning 'num_channel_ios' not described

Fix the below build warning:
warning: Function parameter or member 'num_channel_ios' not described
in 'rz_mtu3_channel_io_map'

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>