i2c: designware_i2c: Rename 'max' speed to 'high' speed

Some SoCs support a higher speed than what is currently called 'max' in
this driver. Rename it to 'high' speed, which is the official name of the
3.4MHz speed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jun Chen <ptchentw@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>