docs: admin-guide: kernel-parameters: Document earlycon options for i.MX UARTs

drivers/tty/serial/imx.c implements these earlycon options.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20200229132750.2783-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>