MIPS: Define __arch_swab64 for all mips r2 cpus

Some CPUs implement mipsr2, but because they are a super-set of mips64r2 do
not define CONFIG_CPU_MIPS64_R2.  Cavium OCTEON falls into this category.
We would still like to use the optimized implementation, so since we have
already checked for CONFIG_CPU_MIPSR2, checking for CONFIG_64BIT instead of
CONFIG_CPU_MIPS64_R2 is sufficient.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>