ARM: shmobile: kzm9g-reference: Add restart callback

Port the sh73a0 restart handling from the kzm9g-legacy board code to the
kzm9g-reference board code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>