drivers: serial: ns16550: Remove DLF & PCP instance count assumptions
drivers: serial: ns16550: Remove DLF & PCP instance count assumptions

Take advantage of DT_INST_FOREACH_STATUS_OKAY() to look for DLF and
PCP properties on any matching nodes with "okay" status.

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

nashif/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr