rockchip: set SYS_NS16550_MEM32 for all SoCs

Add back part of patch send out as
'rockchip: enable SYS_NS16550 for all SoCs by default' that seems to have
gotten lost when it got merged to set SYS_NS16550_MEM32.

Allows serial output to work on tinker-rk3288 again after
c3c0331d.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>