MIPS: Make the kernel arguments from dtb available

MIPS: Make the kernel arguments from dtb available

Similar to how arm allows using selecting between bootloader arguments,
dtb arguments and both, allow to select them on mips. But since we have
less control over the place of the dtb do not modify it but instead use
the boot_command_line for merging them.

The default is "use bootloader arguments" to keep the current behaviour
as default.

Signed-off-by: Jonas Gorski <[email protected]>
Cc: [email protected]
Cc: Kevin Cernekee <[email protected]>
Cc: Florian Fainelli <[email protected]>
Cc: Zubair Lutfullah Kakakhel <[email protected]>
Cc: James Hogan <[email protected]>
Cc: John Crispin <[email protected]>
Cc: Ganesan Ramalingam <[email protected]>
Cc: Jayachandran C <[email protected]>
Cc: Andrew Bresticker <[email protected]>
Cc: James Hartley <[email protected]>
Patchwork: https://patchwork.linux-mips.org/patch/11284/
Signed-off-by: Ralf Baechle <[email protected]>