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>