Platform/ARM: Update Readme.md

The repo with the Visual Studio support no longer exists.
fiptool from the prebuilt_tools repo doesn't work due to a missing
dependency on libcrypto.so.1.0.0, so tell users to build it from the
trusted-firmware-a repo instead.
There's a newer version of fvp-uefi.zip that was released in 2020.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Chris Jones <christopher.jones@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>