Some UART Controllers (Synopsys DesignWare 8250) will trigger spurious interrupts when the Line Control Register (LCR) is set while the UART is busy. This patch provides the option (16550_WAIT_LCR) to wait for UART until it's not busy, before setting the LCR. (16550_WAIT_LCR is disabled by default)
This patch fixes the spurious UART interrupts for the upcoming port of NuttX to StarFive JH7110 SoC (with Synopsys DesignWare 8250 UART). [The patch is explained here](https://lupyuen.github.io/articles/plic#appendix-fix-the-spurious-uart-interrupts)
drivers/serial/uart_16550.c: If 16550_WAIT_LCR is enabled, wait until UART is not busy before setting LCR
include/nuttx/serial/uart_16550.h: Define the UART Status Register (USR) for checking if UART is busy
drivers/serial/Kconfig-16550: Added option 16550_WAIT_LCR to 16550 UART Config, disabled by default
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
