ARM: ux500: reform Ux500 family names

Counting the U9540 and the new U8540 as a U8500 family member
does not work. Instead, split the function in two:

cpu_is_u8500_family() covering U8500 and U8520
cpu_is_ux540_family() covering U9540 and U8540

This works much better in practice. Update users to keep the
same behaviour.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>