arm64: mvebu: Rename the db-88f3720 to armada-37xx platform

Modify the file names and deifinitions relater to Marvell
db-77f3720 board support. Convert these names to more generic
armada-37xx platform for future addition of more boards
based on the same SoC family.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Igal Liberman <igall@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>