MIPS: ath79: Read the initrd address from the firmware environment

Allow loading an initrd passed by the firmware.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8354/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>