crypto: ppc/curve25519 - Low-level primitives for ppc64le

Use the perl output of x25519-ppc64.pl from CRYPTOGAMs
(see https://github.com/dot-asm/cryptogams/) and added four
supporting functions, x25519_fe51_sqr_times, x25519_fe51_frombytes,
x25519_fe51_tobytes and x25519_cswap.
Signed-off-by: Danny Tsen <dtsen@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>