presets: Rename fvp -> an552 and improve descriptions

The suffix `-fvp` of the presets is inaccurate, because what gets built
can run on both FVP and actual hardware/FPGA if they are the same device
model. There also exist FVP models that emulate different devices, and
this suffix doesn't allow any destinction. So we rename the suffix to
`-an552` which is more precise.

Similarly, "Fetch ... and MCU-Driver-Reference-Platforms-for-Arm" is not
precise enough because support for more device models will be added to
the HAL implementation repository, and we will need different presets in
order to build examples for different devices. Instead, we explictly
mention what device we build examples for, e.g. "Build ...
for Corstone-300 AN552".

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>