i2c: add generic routine to parse DT for timing information

Inspired from the i2c-rk3x driver (thanks guys!) but refactored and
extended. See built-in docs for further information.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>