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>

Documentation/devicetree/bindings/serial/nxp,sc16is7xx.txt

@@ -12,6 +12,8 @@ Required properties:
 - reg: I2C address of the SC16IS7xx device.
 - interrupts: Should contain the UART interrupt
 - clocks: Reference to the IC source clock.
+	OR (when there is no clock provider visible to the platform)
+- clock-frequency: The source clock frequency for the IC.
 
 Optional properties:
 - gpio-controller: Marks the device node as a GPIO controller.