MIPS: Make MIPS_CMDLINE_DTB default

Seval of-enabled machines (bmips, lantiq, xlp, pistachio, ralink) copied
the arguments from dtb to arcs_command_line to prevent the kernel from
overwriting them.

Since there is now an option to keep the dtb arguments, default to the
new option remove the "backup" to arcs_command_line in case of USE_OF is
enabled, except for those platforms that still take the bootloader
arguments or do not use any at all.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: Ganesan Ramalingam <ganesanr@broadcom.com>
Cc: Jayachandran C <jchandra@broadcom.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: James Hartley <james.hartley@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/11285/


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