drivers: serial: ns16550: Remove hard-coded max instance count

Instead of having a hard-coded maximum instance count, introduce a
Kconfig variable for it. The inclusion of the per-instance header
files is solved by having them chain-include each other with a
pre-processor condition that checks if the current header file is the
last one or not.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>