ARM: ux500: Enable SDI4 port on the u9540 when booting with Device Tree

Here we add the device node for the SDI4 (MMC) port to the u9540
Device Tree source file. This will allow successful probing of
the internal MMC storage device when booting with DT enabled.

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