uart: ns16550: simplify DLF and PCP related macros

This simplifies DLF and PCP enabling devicetree macros with
DT_ANY_INST_HAS_PROP_STATUS_OKAY() instead of the complicated
ones.

Also, this moves the macro to initialize struct elements into
the struct initializer itself. This makes it clearer on which
element is being initialized directly inside the struct
initializer instead of having to do mental macro trampoline
to find the correct macro.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>