MIPS: ath79: Use the firmware lib to parse the kernel command line

No need to duplicate code that is available in the firmware library.
It also give us access to the firmware environment which is needed
to read the initrd address and size.

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/8353/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>