arm64: juno: Add APB registers and LEDs using syscon

This defines the Juno "APB system registers" as a syscon device,
and all the LEDs controlled by the APB system registers right
below it using the syscon LEDs driver on top of syscon. Define
LED0 for heartbeat, LED1 for MMC0 activity and the following
four LEDs indicating CPU activity using the Linux-specific
DT bindings for triggers.

This is the pattern and same drivers as used on the legacy
platform device trees for the ARM Integrators and the RealView
PB1176.

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