dt-bindings: sc16is7xx: Add alternative clock-frequency property

For the platforms which have no clock provider for the sc16is7xx type of UART,
introduce an alternative clock-frequency property which would be used instead.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>