ARM: ux500: add PERF_EVENTS to defconfig

The Ux500 has a perfectly working PMU so let's put the perf
events into the defconfig. This will activate HW_PERF_EVENTS
by default.

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