OvmfPkg: QemuBootOrder: lower the required number of OFW nodes to 2

OvmfPkg: QemuBootOrder: lower the required number of OFW nodes to 2

2 nodes in an OpenFirmware device path are sufficient for the generic
check at the beginning of TranslateOfwNodes(). The driver specific
branches check for the necessary nodes individually.

The number of nodes saved for examination is unchanged.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13800 6f19259b-4bc3-4df7-8a09-765794883524