i2c: add support for filters optional properties

i2c-digital-filter-width-ns:
This optional timing property specifies the width of the spikes on the i2c
lines (in ns) that can be filtered out by built-in digital filters which are
embedded in some i2c controllers.
i2c-analog-filter-cutoff-frequency:
This optional timing property specifies the cutoff frequency of a low-pass
analog filter built-in i2c controllers. This low pass filter is used to filter
out high frequency noise on the i2c lines. Specified in Hz.
Include these properties in the timings structure and read them as integers.

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