ARM: shmobile: mackerel: Remove ZBOOT code

Remove ZBOOT support code for Mackerel. With this patch in place
it is no longer possible to boot a self-contained kernel directly
from the reset vector. ZBOOT is still supported on kzm9g.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>