ARM: zImage: don't define unused symbol initrd_phys

The only user of initrd_phys is arch/arm/boot/bootp/init.S which still
gets the value passed to.

Acked-by: Eric Miao <eric.miao@canonical.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>