serial: imx: Improve Kconfig text

Some improvements in the Kconfig text:

- Vendor is Freescale now
- Provide a better example for the console parameter, which is something like
console=ttymxc0
- Do not pass the names of ancient bootloaders

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>