ARM: ux500: Only configure wake-up reasons on ux500 based platforms

Multiplatform calls all enabled platforms' initcalls. In the
ux500_idle_init() initcall we call into the DBx500-PRCMU which in turn
executes some ux500 specific register reads/writes. When running on
some !ux500 platforms this ends up causing a kernel Oops. This patch
ensures the PRCMU call is only invoked when running on ux500 based
platforms.

Reported-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>