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>
diff --git a/doc/board/emulation/qemu-arm.rst b/doc/board/emulation/qemu-arm.rst
index b42d924..7291fa4 100644
--- a/doc/board/emulation/qemu-arm.rst
+++ b/doc/board/emulation/qemu-arm.rst
@@ -113,7 +113,7 @@
 
 Enable the TPM on U-Boot's command line with::
 
-    tpm2 startup TPM2_SU_CLEAR
+    tpm autostart
 
 Debug UART
 ----------