i2c: designware: Look for *CNT values in DT

The i2c-designware driver supports reading precise timing values from
ACPI, but the Device Tree support relies on a combination of standard
rise and fall times and hard-coded minimum timings. The result of this
is that it is difficult to get optimum timings, particularly given that
the values are bus speed-specific and only one set can be stored in
DT at a time.

Add support for initialisation from DT that is similar to that for
ACPI.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>