docs: serial: move it to the driver-api

Documentation/driver-api/index.rst

Documentation/driver-api/serial/cyclades_z.rst

Documentation/driver-api/serial/driver.rst

Documentation/driver-api/serial/index.rst

Documentation/driver-api/serial/moxa-smartio.rst

Documentation/driver-api/serial/n_gsm.rst

Documentation/driver-api/serial/rocket.rst

Documentation/driver-api/serial/serial-iso7816.rst

Documentation/driver-api/serial/serial-rs485.rst

Documentation/driver-api/serial/tty.rst

MAINTAINERS

drivers/tty/Kconfig

drivers/tty/serial/ucc_uart.c

include/linux/serial_core.h