serial: imx: Add more comments on port lock status

Comments regarding status of port.lock on internal functions is useful when
reviewing correct handling of registers that must be protected by this
lock.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Link: https://lore.kernel.org/r/20240913-serial-imx-lockfix-v1-2-4d102746c89d@geanix.com


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