powerpc/rtas: Make enter_rtas() private

There are no longer any users of enter_rtas() outside of rtas.c, so make
it "private", by moving the declaration inside rtas.c. Hopefully this
will encourage people to use one of the wrappers which takes the sharp
edges off the RTAS calling sequence.

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