samd/boards/SPARKFUN_SAMD51_THING_PLUS: Add board files for Thing Plus.

That device uses an SAMD51J20 MCU with 256k RAM and 1024k flash.

ports/samd/boards/SPARKFUN_SAMD51_THING_PLUS/board.json

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+{
+    "deploy": [
+        "../deploy.md"
+    ],
+    "docs": "",
+    "features": [
+        "Battery Charging",
+        "Breadboard Friendly",
+        "Micro USB",
+        "QWIIC",
+        "SPI Flash"
+    ],
+    "images": [
+        "sparkfun_samd51_thing_plus.jpg"
+    ],
+    "mcu": "samd51",
+    "product": "Sparkfun SAMD51 Thing Plus",
+    "thumbnail": "",
+    "url": "https://www.sparkfun.com/products/14713",
+    "vendor": "Sparkfun"
+}

ports/samd/boards/SPARKFUN_SAMD51_THING_PLUS/mpconfigboard.h

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+#define MICROPY_HW_BOARD_NAME       "Sparkfun SAMD51 Thing Plus"
+#define MICROPY_HW_MCU_NAME         "SAMD51J20A"
+
+#define MICROPY_HW_XOSC32K          (1)

ports/samd/boards/SPARKFUN_SAMD51_THING_PLUS/mpconfigboard.mk

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+MCU_SERIES = SAMD51
+CMSIS_MCU = SAMD51J20A
+LD_FILES = boards/samd51x20a.ld sections.ld
+TEXT0 = 0x4000
+
+# The ?='s allow overriding in mpconfigboard.mk.
+# MicroPython settings
+MICROPY_VFS_LFS1 ?= 1

ports/samd/boards/SPARKFUN_SAMD51_THING_PLUS/pins.csv

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+# Pin rows contain Pin number and pin name.
+# Pin rows start with PIN_
+# LED rows start with LED_
+# If the pin name is omitted, the pin number is added as name.
+# Rows for empty entries have to start with '-'
+# Empty lines and lines not starting with PIN_ or LED_ are ignored
+
+PIN_PA13,D0
+PIN_PA12,D1
+PIN_PB23,RXD
+PIN_PB22,TXD
+PIN_PA06,D4
+PIN_PA15,D5
+PIN_PA20,D6
+PIN_PA21,D7
+-
+PIN_PA07,D9
+PIN_PA18,D10
+PIN_PA16,D11
+PIN_PA19,D12
+PIN_PA17,D13
+PIN_PA02,A0
+PIN_PB08,A1
+PIN_PB09,A2
+PIN_PA04,A3
+PIN_PA05,A4
+PIN_PB02,A5
+PIN_PA22,SDA
+PIN_PA23,SCL
+PIN_PB12,MOSI
+PIN_PB11,MISO
+PIN_PB13,SCK
+PIN_PA08,FLASH_MOSI
+PIN_PA09,FLASH_SCK
+PIN_PA10,FLASH_CS
+PIN_PA11,FLASH_MISO
+PIN_PA30,SWDCLK
+PIN_PA31,SWDIO
+
+LED_PA17,LED
+LED_PB03,RXLED
+LED_PA27,TXLED

ports/samd/boards/samd51x20a.ld

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+/*
+    GNU linker script for SAMD51x20
+*/
+
+/* Specify the memory areas */
+MEMORY
+{
+    FLASH (rx)  : ORIGIN = 0x00004000, LENGTH = 1024K - 16K
+    RAM (xrw)   : ORIGIN = 0x20000000, LENGTH = 256K
+}
+
+/* Top end of the stack, with room for double-tap variable */
+_estack = ORIGIN(RAM) + LENGTH(RAM) - 8;
+_sstack = _estack - 16K;
+
+_oflash_fs = ORIGIN(FLASH) + 384K - 16K;
+_sflash_fs = LENGTH(FLASH) - 384K + 16K - 1;
+
+_sheap = _ebss;
+_eheap = _sstack;