Add BBRAM and eFUSE usage to the Xilinx Versal Virt board
document.

Signed-off-by: Tong Ho <tong.ho@xilinx.com>
Message-id: 20210917052400.1249094-10-tong.ho@xilinx.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>