serial: sc16is7xx: Respect clock-frequency property

If the property is provided and there are no other possibilities to detect
UART clock frequency, use it as a fallback.

Tested-By: Georgii Staroselskii <georgii.staroselskii@emlid.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>