ARM: 7022/1: allow to detect conflicting zreladdrs

Boards used to specify zreladdr in their Makefile.boot with
zreladdr-y := x, so conflicting zreladdrs were silently overwritten.
This patch changes this to zreladdr-y += x, so that we end
up with multiple words in zreladdr in such a case. We can
detect this later and complain if necessary.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>