marvell: Move BLE from external repo to the platform folder

The BLE is the pre-TF-A boot stage required by Marvell Armada
BootROM for bringing up DRAM and allow the boot image copy to it.
Since this is not a standard boot level and only uses the TF-A
as a build environment, it was introduced out of source tree.
However it turns out that such remote location introduces additional
complexity to the upstream TF-A build process.
In order to simplify the build environment the BLE source folder
is relocated from the external repository to A8K platform directory.
The build documentation is updated accordingly.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>