plat/arm: Remove board/ directory

The board/ directory does not actually bring any benefit. There's no
need to separate the board level from any other level. Thus, this
patch flattens the Arm platform tree like so:

plat/arm/
|- common
|- fvp
|- juno
|- sgi
|   |- common
|   |- sgi575

Change-Id: I10e4af7594bb555d912222a881d95eae9864637e
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
26 files changed