docs/system/arm/aspeed: add the sanmiguel-bmc machine

The Facebook SanMiguel BMC machine is supported by hw/arm but has never
been listed in the Aspeed documentation. Add it to the AST2600 machine
table.

Reviewed-by: Jamin Lin <jaminlin1207@gmail.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Emmanuel Blot <emmanuel.blot@free.fr>
Link: https://lore.kernel.org/qemu-devel/20261015-bmc-minerva-v3-2-7d50c8a108e5@free.fr
Signed-off-by: Cédric Le Goater <clg@redhat.com>

docs/system/arm/aspeed.rst

diff --git a/docs/system/arm/aspeed.rst b/docs/system/arm/aspeed.rst
index d3749c0..2e23123 100644
--- a/docs/system/arm/aspeed.rst
+++ b/docs/system/arm/aspeed.rst
@@ -34,6 +34,7 @@
 - ``bletchley-bmc``        Facebook Bletchley BMC
 - ``fby35-bmc``            Facebook fby35 BMC
 - ``catalina-bmc``         Facebook Catalina BMC
+- ``sanmiguel-bmc``        Facebook SanMiguel BMC
 - ``anacapa-bmc``          Facebook Anacapa BMC
 - ``gb200nvl-bmc``         Nvidia GB200nvl BMC