arm: set initrd_start/initrd_end for fdt scan

In order to unify the initrd scanning for DT across architectures, make
arm set initrd_start and initrd_end instead of the physical addresses.
This is aligned with all other architectures.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Acked-by: Grant Likely <grant.likely@linaro.org>