ARM: ux500: Disable SMSC911x platform code registration when DT is enabled

Now the SCMC911x is correctly enabled in Device Tree, there is no need
to continue registering it from platform code. In fact, if we continue
doing so, the system will throw an error on boot.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>