lib/mpi: Extend the MPI library

Expand the mpi library based on libgcrypt, and the ECC algorithm of
mpi based on libgcrypt requires these functions.
Some other algorithms will be developed based on mpi ecc, such as SM2.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Tested-by: Xufeng Zhang <yunbo.xufeng@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>