PSA unified driver interface

Working draft of the PSA cryptography unified interface specification.
Eventually this document will be under Arm PSA architecture ownership,
but for the time being this draft is maintained in Mbed TLS.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/docs/proposed/Makefile b/docs/proposed/Makefile
index 97f288a..cf656dd 100644
--- a/docs/proposed/Makefile
+++ b/docs/proposed/Makefile
@@ -3,6 +3,7 @@
 default: all
 
 all_markdown = \
+	       psa-driver-interface.md \
 	       # This line is intentionally left blank
 
 html: $(all_markdown:.md=.html)