powerpc/mm: Rename hpte_init_lpar() and move the fallback to a header

hpte_init_lpar() is part of the pseries platform, so name it as such.

Move the fallback implementation for when PSERIES=n into the header,
dropping the weak implementation. The panic() is now handled by the
calling code.

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