ArmPkg/BdsLib: Update the size of the Device Tree before booting Linux

ArmPkg/BdsLib: Update the size of the Device Tree before booting Linux

The memory reserved to load the device tree is over-allocated when
UEFI loads the blob to update the device tree with additional
information such as the command line, the initrd, etc.

This change ensures the total size declared by the device tree matches
the actual data (and not the over-allocated size value).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16517 6f19259b-4bc3-4df7-8a09-765794883524