powerpc/64: Rename entry_64.S to prom_entry_64.S

This file contains only the enter_prom implementation now.
Trim includes and update header comment while we're here.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606132447.315714-7-npiggin@gmail.com