docs: sbsa: update specs, add dt note

docs: sbsa: update specs, add dt note

Hardware of sbsa-ref board is nowadays defined by both BSA and SBSA
specifications. Then BBR defines firmware interface.

Added note about DeviceTree data passed from QEMU to firmware. It is
very minimal and provides only data we use in firmware.

Added NUMA information to list of things reported by DeviceTree.

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-id: 20240328163851.1386176-1-marcin.juszkiewicz@linaro.org
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>