ARM: ux500: Enable all MMC devices on the u9540 when booting with DT

The u9540 supports 3 MMC devices. This patch enables two of them
and updates the configuration of the already enabled SDI4 port.

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