ArmPlatformPkg/Bds: Fixed adding support for new boot entry when the generated Device...

ArmPlatformPkg/Bds: Fixed adding support for new boot entry when the generated DevicePath is bigger than one node

The initial support was only considering the added Device Path will be a single node.
TFTP for instance requires two new nodes on the top of the ethernet Device Path; a first
one for the IP address of the server and a second one for the file to download.

This change replace the return argument from a DevicePath node by a DevicePath.
It means the End Device Path node is now required.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14194 6f19259b-4bc3-4df7-8a09-765794883524