MIPS: select CPU_MIPS64 for remaining MIPS64 CPUs

CPU_MIPS64 is supposed to be selected for CPUs that implement a revision
of the MIPS64 ISA. While it contains the generic ones, it forgot about
Octeon and Loongson in its list, which are indeed MIPS64 processors.
This commit adds these missing CPUs to the auto-selection list.

Cc: Maciej W. Rozycki <macro@orcam.me.uk>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: George Cherian <gcherian@marvell.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>