ARM: dts: bcm2712-rpi-5-b: Add act_led_gpio

The Pi 5 DTB was lacking the act_led_gpio parameter, and
act_led_activelow as implemented the old way. Fix both.

See: https://forums.raspberrypi.com/viewtopic.php?t=364719



Signed-off-by: Phil Elwell <phil@raspberrypi.com>