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
b2c1930825