infra/rdn2: improve the steps listed to obtain the FVP

Add detailed steps on obtaining the FVP for RD-N2, RD-N2-Cfg1 and
RD-N2-Cfg2 platforms. The name of the FVP is also stated to help
identify the FVP executable in the downloaded FVP package.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I3f4f704b20191ba4b1edf513b132b9c3fe3cc779