Remove i.MX7 IOMUX GPR device from i.MX6UL

Remove i.MX7 IOMUX GPR device from i.MX6UL

i.MX7 IOMUX GPR device is not equivalent to i.MX6UL IOMUXC GPR device.
In particular, register 22 is not present on i.MX6UL and this is actualy
The only register that is really emulated in the i.MX7 IOMUX GPR device.

Note: The i.MX6UL code is actually also implementing the IOMUX GPR device
as an unimplemented device at the same bus adress and the 2 instantiations
were actualy colliding. So we go back to the unimplemented device for now.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 48681bf51ee97646479bb261bee19abebbc8074e.1692964892.git.jcd@tribudubois.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>