drivers/serial/uart_16550.c: Support 16550 auto hardware flow control
drivers/serial/uart_16550.c: Add configuration option CONFIG_16550_SUPRESS_INITIAL_CONFIG. This is identical to the standard configuration in arch/Kconfig CONFIG_SUPPRESS_UART_CONFIG, but with scope of only the 16550 driver.
drivers/serial/uart_16550.c: Add a configuration, analogous to the STM32 configuration option, to suppress the NuttX standard re-ordering for /dev/ttySN for special case of the 16550 UART.
config/serial: UART 16550: Add CONFIG_SERIAL_UART_ARCH_MMIO option so the a memory mapped device doesn't need to provide uart_getreg() and uart_putreg() implementations.
u16550_txempty() should check UART_LSR_TEMT to avoid some data left in the transmit FIFO
