crypto: ecdsa - Support P1363 signature decoding

Alternatively to the X9.62 encoding of ecdsa signatures, which uses
ASN.1 and is already supported by the kernel, there's another common
encoding called P1363.  It stores r and s as the concatenation of two
big endian, unsigned integers.  The name originates from IEEE P1363.

Add a P1363 template in support of the forthcoming SPDM library
(Security Protocol and Data Model) for PCI device authentication.

P1363 is prescribed by SPDM 1.2.1 margin no 44:

   "For ECDSA signatures, excluding SM2, in SPDM, the signature shall be
    the concatenation of r and s.  The size of r shall be the size of
    the selected curve.  Likewise, the size of s shall be the size of
    the selected curve.  See BaseAsymAlgo in NEGOTIATE_ALGORITHMS for
    the size of r and s.  The byte order for r and s shall be in big
    endian order.  When placing ECDSA signatures into an SPDM signature
    field, r shall come first followed by s."

Link: https://www.dmtf.org/sites/default/files/standards/documents/DSP0274_1.2.1.pdf


Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>