arm64: add LEDs and some trigger support to defconfig

Since many boards for ARM, experimental or server alike, will
feature LEDs, let's include LED class and trigger support for
heartbeat and CPU in the defconfig. Many systems (such as the
ARM Juno) will use a system controller "syscon" to access the
LED registers, so include support for this as well.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>