doc: qemu: switch swtpm instruction to 'tpm autostart'

We don't have a documentation page for our TPM subsystem.  I plan
on sending one in the future,  but in the meantime document the
new 'tpm autostart' command in the QEMU instructions while using
a SWTPM

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