Adjust function definitions in mmu_radix.c to avoid clang 15 warnings

Adjust function definitions in mmu_radix.c to avoid clang 15 warnings

With clang 15, the following -Werror warnings are produced:

    sys/powerpc/aim/mmu_radix.c:786:20: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    mmu_radix_tlbie_all()
                       ^
                        void
    sys/powerpc/aim/mmu_radix.c:3615:15: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    mmu_radix_init()
                  ^
                   void
    sys/powerpc/aim/mmu_radix.c:6081:20: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    mmu_radix_scan_init()
                       ^
                        void

This is because mmu_radix_tlbie_all(), mmu_radix_init(), and
mmu_radix_scan_init() are declared with (void) argument lists, but
defined with empty argument lists. Make the definitions match the
declarations.

MFC after:	3 days