ARM: ux500: read the correct soc_id number

Fix db8500_read_soc_id() to read all five soc_id number locations
instead of repeating the second one two times.

Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>