ARM: ux500: Enable the external bus with Device Tree

To allow for proper initialisation of the SMSC9115 Ethernet chip
and other devices which may be connected to the 'external bus',
Device Tree must first enable and traverse though it. This patch
enables the 'external bus' which will facilitate further
initialisation of its suborantate DT nodes.

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