Update opensbi submodule to upstream master

Updating from commit id e561c63:
2019-10-02 17:03:58 +0530 - (lib: Fix coldboot race condition observed on emulators/simulators)

to commit id 215421c:
2019-11-11 16:40:34 -0800 - (lib: Remove date and time from init message)

This brings in 13 new commits and allows reproducible builds with
opensbi.

Change-Id: I0fb9e0921b017822defa8b56df5a0f3e014d7f33
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36866
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

3rdparty/opensbi

@@ -1 +1 @@
-Subproject commit e561c6303639ed510183da25d3d54555a53371c9
+Subproject commit 215421ca610a64b8ec188c96ea8588ae2de41fb7