ARM: ux500: use CLK_OF_DECLARE()

The ux500 DT support predates the CLK_OF_DECLARE macro and calls
directly into the clk driver from platform code.

Converting this to CLK_OF_DECLARE makes the code much nicer and
similar to how modern platforms do it today. It also removes the
last user of cpu_is_u8500_family() etc.

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