dt-bindings: i2c: add bindings for i2c analog and digital filter

Some i2c controllers have a built-in digital or analog filter.
This is specifically required depending on the hardware PCB/board.
Some controllers also allow specifying the maximum width of the
spikes that can be filtered for digital filter. The width length can be
specified in nanoseconds.
Analog filters can be configured to have a cutoff frequency (low-pass filter).
This frequency can be specified in Hz.
Added an optional property for such types of analog filters.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c.txt

@@ -55,6 +55,24 @@ wants to support one of the below features, it should adapt the bindings below.
 	Number of nanoseconds the SDA signal takes to fall; t(f) in the I2C
 	specification.
 
+- i2c-analog-filter
+	Enable analog filter for i2c lines.
+
+- i2c-digital-filter
+	Enable digital filter for i2c lines.
+
+- i2c-digital-filter-width-ns
+	Width of spikes which can be filtered by digital filter
+	(i2c-digital-filter). This width is specified in nanoseconds.
+
+- i2c-analog-filter-cutoff-frequency
+	Frequency that the analog filter (i2c-analog-filter) uses to distinguish
+	which signal to filter. Signal with higher frequency than specified will
+	be filtered out. Only lower frequency will pass (this is applicable to
+	a low-pass analog filter). Typical value should be above the normal
+	i2c bus clock frequency (clock-frequency).
+	Specified in Hz.
+
 - interrupts
 	interrupts used by the device.