serial: fsl_lpuart: documented the clock requirement.

This is a correction to the documentation. It was previously required but not documented.

Signed-off-by: Yuan Yao <yao.yuan@freescale.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>