hw/arm: xlnx-zcu102: Add Xilinx BBRAM device

hw/arm: xlnx-zcu102: Add Xilinx BBRAM device

Connect the support for Xilinx ZynqMP Battery-Backed RAM (BBRAM)

The command argument:
  -drive if=pflash,index=2,...
Can be used to optionally connect the bbram to a backend
storage, such that field-programmed values in one
invocation can be made available to next invocation.

The backend storage must be a seekable binary file, and
its size must be 36 bytes or larger. A file with all
binary 0's is a 'blank'.

Signed-off-by: Tong Ho <tong.ho@xilinx.com>
Message-id: 20210917052400.1249094-8-tong.ho@xilinx.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>