mips: add SYS_HAS_CPU_MIPS64_R5 config for MIPS Release 5 support

Commit ab7c01fd ("mips: Add MIPS Release 5 support") adds the two
configs CPU_MIPS32_R5 and CPU_MIPS64_R5, which depend on the corresponding
SYS_HAS_CPU_MIPS32_R5 and SYS_HAS_CPU_MIPS64_R5, respectively.

The config SYS_HAS_CPU_MIPS32_R5 was already introduced with commit
c5b36783 ("MIPS: Add support for XPA."); the config
SYS_HAS_CPU_MIPS64_R5, however, was never introduced.

Hence, ./scripts/checkkconfigsymbols.py warns:

  SYS_HAS_CPU_MIPS64_R5
  Referencing files: arch/mips/Kconfig, arch/mips/include/asm/cpu-type.h

Add the definition for config SYS_HAS_CPU_MIPS64_R5 under the assumption
that SYS_HAS_CPU_MIPS64_R5 follows the same pattern as the existing
SYS_HAS_CPU_MIPS32_R5 and SYS_HAS_CPU_MIPS64_R6.

Fixes: ab7c01fd ("mips: Add MIPS Release 5 support")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>