docs: add some notes on the sbsa-ref machine

We should at least document what this machine is about.

Reviewed-by: Graeme Gregory <graeme@nuviainc.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20201104165254.24822-1-alex.bennee@linaro.org
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Shashi Mallela <shashi.mallela@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
[PMM: fixed filename mismatch]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/sbsa.rst

@@ -0,0 +1,32 @@
+Arm Server Base System Architecture Reference board (``sbsa-ref``)
+==================================================================
+
+While the `virt` board is a generic board platform that doesn't match
+any real hardware the `sbsa-ref` board intends to look like real
+hardware. The `Server Base System Architecture
+<https://developer.arm.com/documentation/den0029/latest>` defines a
+minimum base line of hardware support and importantly how the firmware
+reports that to any operating system. It is a static system that
+reports a very minimal DT to the firmware for non-discoverable
+information about components affected by the qemu command line (i.e.
+cpus and memory). As a result it must have a firmware specifically
+built to expect a certain hardware layout (as you would in a real
+machine).
+
+It is intended to be a machine for developing firmware and testing
+standards compliance with operating systems.
+
+Supported devices
+"""""""""""""""""
+
+The sbsa-ref board supports:
+
+  - A configurable number of AArch64 CPUs
+  - GIC version 3
+  - System bus AHCI controller
+  - System bus EHCI controller
+  - CDROM and hard disc on AHCI bus
+  - E1000E ethernet card on PCIe bus
+  - VGA display adaptor on PCIe bus
+  - A generic SBSA watchdog device
+

docs/system/target-arm.rst

@@ -79,6 +79,7 @@ undocumented; you can get a complete list by running
    arm/mps2
    arm/musca
    arm/realview
+   arm/sbsa
    arm/versatile
    arm/vexpress
    arm/aspeed