Provide an additional pair of #defines, MBEDTLS_SHA256_USE_A64_CRYPTO_IF_PRESENT
and MBEDTLS_SHA256_USE_A64_CRYPTO_ONLY. At most one of them may be
specified. If used, it is necessary to compile with -march=armv8-a+crypto.
The MBEDTLS_SHA256_PROCESS_ALT and MBEDTLS_SHA256_ALT mechanisms
continue to work, and are mutually exclusive with A64_CRYPTO.
There should be minimal code size impact if no A64_CRYPTO option is set.
Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
