include RTC fix for SAMd51 in circuitpython build

mrmcwethy · mrmcwethy · commit d392f01f2ca8 · 2018-06-29T05:45:38.000-07:00
diff --git a/ports/atmel-samd/asf4 b/ports/atmel-samd/asf4
@@ -1 +1 @@
-Subproject commit cdde379f78989436590a556cadbc0e201666ee04
+Subproject commit 88a67f9c83eeb225fe65f596078f8aaa3f6de1ec
