ARM: ux500: Remove duplicate DT_MACHINE_START - make u8500 for generic

The u8500 and u9540 DT_MACHINE_START calls are almost identical
at the moment, referencing exactly the same function pointers.
Until this changes we may as well convert the original one to
be more generic and use that instead.

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