build: deprecate Arm rdn1edge and sgi575 FVP platforms

Arm has decided to deprecate the sgi575 and rdn1edge platforms.
The development of software and fast models for these platforms
has been discontinued. rdn1edge platform has been superseded by the
rdn2 platform, which is already supported in TF-A and CI work is
underway for this platform.

Change-Id: If2228fb73549b244c3a5b0e5746617b3f24fe771
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>