ARM: bcm2835: Add Raspberry Pi's ACT LED to DT

The Raspberry Pi board has one GPIO-controlled LED labeled "ACT". Add it
to the DT via the gpio-leds driver, so users can control it from
userspace. If CONFIG_LEDS_TRIGGER_HEARTBEAT is set, the LED will also
signal some sign of life.

The GPIO circuitry is low-active. And as the bootloader may decide to
switch the LED on at boot time, the default state is 'keep'.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>