ns16650: Correct CONFIG_SYS_NS16550_MEM32 platforms

There are currently no platform that are both CONFIG_SYS_NS16550_MEM32
and not (per how the logic was prior to being broken in 0478dac6
("kbuild: Remove uncmd_spl logic")) enabled in CONFIG_DM_SERIAL. We drop
this line out now so that platforms which do use
CONFIG_SYS_NS16550_MEM32 and depending on stage may or may not have
DM_SERIAL set.

Fixes: 0478dac6 ("kbuild: Remove uncmd_spl logic")
Reported-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Quentin Schulz <quentin.schulz@theobroma-systems.com> #on Ringneck PX30, Puma RK3399