arm64: set initrd_start/initrd_end for fdt scan

In order to unify the initrd scanning for DT across architectures, make
arm64 use initrd_start and initrd_end instead of the physical addresses.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org