arm/edk2-firmare: use gitsm fetcher

The EDK2 repository contains a number of submodules and some builds need
them, so use the gitsm fetcher to ensure they're always fetched.

Change-Id: I63cf20900b3fa636f409926222c715b7f6a7bb89
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>