ARM: ux500: consolidate soc_device code in id.c

Nothing else uses the global dbx500_asic_id structure, so
we can merge the two small files that reference it into one.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>