MIPS: add board qemu-mips64 support

Both big-endian and little-endian are tested with below commands:
Rom version: (Default, Now we config it as rom version)
qemu-system-mips64el -M mips -bios u-boot.bin -cpu MIPS64R2-generic -nographic
qemu-system-mips64 -M mips -bios u-boot.bin -cpu MIPS64R2-generic -nographic
Ram version:
qemu-system-mips64el -M mips -cpu MIPS64R2-generic -kernel u-boot -nographic
qemu-system-mips64 -M mips -cpu MIPS64R2-generic -kernel u-boot -nographic

Signed-off-by: Zhizhou Zhang <etou.zh@gmail.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>