serial: 8250_rt288x: Name non-standard divisor latch reg

Instead of a literal, add proper name for the non-standard divisor
latch register.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230511121029.13128-6-ilpo.jarvinen@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>