openrisc: restart: Call common handlers before hanging

Currently openrisc will print a message and then hang in an infinite
loop when rebooting.

This patch adopts some code from ARM, which calls the common restart
infrastructure and hangs after a small delay if the restart infra
doesn't do anything.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stafford Horne <shorne@gmail.com>