ARM: ux500: create MCDE node to collect resources

As we need to connect resources such as pin mappings and clocks
when deleting board files, we create a MCDE node even though there
is no driver for it. As it is only using standard bindings right
now, this does not matter much. When a proper driver is written
for the MCDE, it can augment this node with custom properties.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>