[MIPS64] Add mips64 target support to build-kernel.sh

Add support for building mips64 Ranchu kernel for
booting the new code base QEMU ranchu virtual board.

docs/ANDROID-KERNEL.TXT updated.

Change-Id: I8146b597ee81137a127ab47ef25bf1713ec36df3
Merge "[MIPS64] Add mips64 target support to build-kernel.sh"