efi_loader: define GUIDS for event groups

Event groups are used to signal multiple events at the same time.
They are identified by GUIDs. This patch provided the predefined
GUIDs of UEFI specification 2.7.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>