firmware: raspberrypi: notify VC4 firmware of a reboot

The firmware-owned GPIO expander on RPi 3 B+ must be in same state
after a reboot as initial power on. Otherwise this would cause a
network boot failure of the BOOTROM. So inform the VC4 firmware to restore
the expander before doing a reboot.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Phil Elwell <phil@raspberrypi.org>
Reviewed-by: Eric Anholt <eric@anholt.net>