ARM: ux500: Remove ux500-musb platform registation when booting with DT

Now the ux500-musb driver has been enabled for Device Tree, there is no
requirement to register it from platform code.

Acked-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>