ARM: ux500: Enable the SMSC9115 on Snowball via Device Tree

Here we split the description of the external-bus@50000000 over
two description files. In the more generic db8500 description file
we only specify the external-bus. Normally this would be used to
communicate with a NOR-flash device. On the Snowball however, the
SMSC9115 Ethernet chip occupies it. In the Snowball board specific
description file is where we actually specify that it is in fact the
Ethernet device which lives here.

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