infra/common: document steps to sync reference software stack

Add a new document that explains the steps to download (sync) the
Neoverse reference design software stack for RD platforms. This
document also explains the steps to install the prerequisite for
building the stack.

Change-Id: I2415562eab59e91c0ca211262fdf6c32b94df1e9
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>