add samd asf4 submodule repo from Adafruit

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "hw/mcu/nordic/nrfx"]
 	path = hw/mcu/nordic/nrfx
 	url = https://github.com/NordicSemiconductor/nrfx.git
+[submodule "hw/mcu/microchip/samd/asf4"]
+	path = hw/mcu/microchip/samd/asf4
+	url = git@github.com:adafruit/asf4.git

hw/mcu/microchip/samd/asf4

@@ -0,0 +1 @@
+Subproject commit d270f79aa16dd8fd4ae3b6c14544283dcb992e9c