powerpc/mm: Define radix_enabled() in one place & use static inline

Currently we have radix_enabled() three times, twice in asm/book3s/64/mmu.h
and then a fallback in asm/mmu.h.

Consolidate them in asm/mmu.h. While we're at it convert them to be
static inlines, and change the fallback case to returning a bool, like
mmu_has_feature().

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>