MIPS: implement device-tree handover to Linux kernel

Add device-tree handover to Linux kernel conforming with MIPS UHI [1].
Register $a0 will be set to the reserved value -2. The address of
the device-tree blob will be stored as KSEG0 address in $a1. $a2 and
$a3 are set to zero.

[1] http://prplfoundation.org/wiki/MIPS_documentation



Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>