serial: max310x: add macro for max number of ports

Add macro to hold the maximum number of UART ports per IC/device.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Link: https://lore.kernel.org/all/ddbc67dd-f8a3-4a6a-954a-bee49260ecab@cesnet.cz/


Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20240118152213.2644269-8-hugo@hugovil.com


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