md5: Use typedef for MD5 context

Use of typedef is beneficial for porting with other crypto libs
without changing the API callers.
Secondly, it is for the code consistency with other digest libs.
SHA1, SHA256 and SHA512 are all using typedef for their context.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>