tpm: Implement tpm_auto_start() for TPMv1.2

Add an implementation of this, moving the common call to tpm_init() up
into the common API implementation.

Add a test.

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>