As the binding for slew-rate is under discussion and seems to need
more tought it will get removed for now, so it doesn't get an offical
release.

Therefore remove it again from the only current user, tz1090.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>