Update vboot submodule pointer

Update the pointer for vboot_reference so it can be used to compile
depthcharge payload on the master branch.

Change-Id: I5fc6e05896d7221a1e48ca86c6b15081488302b5
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39840
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

3rdparty/vboot

@@ -1 +1 @@
-Subproject commit 5059062dd352e3864fb68f8a061e87bd7055d12a
+Subproject commit 46ff62c31c1d815196fa990e613e1e6e229f8ce2