Add board support for Adafruit Grand Central M4 (SAMD51) (#1714)

machine/grandcentral-m4: implementation for Adafruit Grand Central M4 board.

targets/atsamd51p20a.json

@@ -0,0 +1,13 @@
+{
+	"inherits": ["cortex-m4"],
+	"build-tags": ["sam", "atsamd51", "atsamd51p20", "atsamd51p20a"],
+	"cflags": [
+		"-Qunused-arguments"
+	],
+	"linkerscript": "targets/atsamd51p20a.ld",
+	"extra-files": [
+		"src/device/sam/atsamd51p20a.s"
+	],
+	"openocd-transport": "swd",
+	"openocd-target": "atsame5x"
+}

targets/atsamd51p20a.ld

@@ -0,0 +1,10 @@
+
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0x00000000+0x4000, LENGTH = 0x00100000-0x4000  /* First 16KB used by bootloader */
+    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 0x00040000
+}
+
+_stack_size = 4K;
+
+INCLUDE "targets/arm.ld"

targets/grandcentral-m4.json

@@ -0,0 +1,9 @@
+{
+    "inherits": ["atsamd51p20a"],
+    "build-tags": ["grandcentral_m4"],
+    "flash-1200-bps-reset": "true",
+    "flash-method": "msd",
+    "msd-volume-name": "GCM4BOOT",
+    "msd-firmware-name": "firmware.uf2",
+    "openocd-interface": "jlink"
+}