ARM: ux500: Introduce cpu_is_u8580()

Detect the subrevision v1 of U8540 and U8580.

DB8580 is the FDSOI variant of DB8540. Some differences
with DB8540 related to clocks will need to be checked with
a cpu_is_u8580() function.

Signed-off-by: Maxime Coquelin <maxime.coquelin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>