Replace the the private field acces of mbedtls_dhm_context with APIs

[gabor-mezei-arm]

As part of #4683, the review of programs for MBEDTLS_PRIVATE field usage concluded that are usage of private fields of mbedtls_dhm_context structure. All occurrence of this usage can be avoided by using APIs.

As part of this issue all occurrence of the 'mbedtls_dhm_context' private filed usage must be replaced with the following APIs:

field	replacing getter APIs	replacing setter APIs
len	mbedtls_dhm_get_len
P, G	mbedtls_dhm_get_value	mbedtls_dhm_set_group

These are the affected files:

• pkey/dh_server.c
• benchmark.c