dt-bindings: serial: document fsl,inverted-tx and -rx options

Add a description for the new fsl,inverted-tx and fsl,inverted-rx
options for the i.MX UART peripheral.

Signed-off-by: George Hilliard <ghilliard@kopismobile.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200226222319.18383-2-ghilliard@kopismobile.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>