ARM: ux500: Remove unused board compatible string

'st-ericsson,u8500' is too much of a broad name to be used for
any of the supported development boards supported by ST-Ericsson.
As such it was never used. It has no place to remain in the board
compatible list, hence why it's being removed.

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