kernel: backport pending fix to select CPU_MIPS64

kernel: backport pending fix to select CPU_MIPS64

The CPU_MIPS64 and CPU_MIPS32 variables are supposed to be able to
distinguish broadly between 64-bit and 32-bit MIPS CPUs. However, they
weren't selected by the specialty CPUs, Octeon and Loongson, which meant
it was possible to hit a weird state of:

   MIPS=y, CONFIG_64BIT=y, CPU_MIPS64=n

This commit rectifies the issue by having CPU_MIPS64 be selected when
the missing Octeon or Loongson models are selected.

In particular, this affects our octeonplus target.

It has been posted to LKML here:
https://lore.kernel.org/linux-mips/20210227122605.2680138-1-Jason@zx2c4.com/

Cc: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Cc: David Bauer <mail@david-bauer.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>