ARM: ux500: Apply other compatible name to the u8540 DTS file

Devices in board DTS files are usually searchable by multiple
compatible strings. In this case the 8540 based SoC should be
obtainable via the chipset [DB8540 & AB8540] name 'u8540' or
the board name 'ccu8540'. It's already possible to obtain the
Device Tree via its chipset name, this patch makes it possible
to get it by its board name too.

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