boards: update nRF51dk board to board scheme v2.

This commit updates nrf51dk_nrf51422 to use board scheme v2, and
thus becomes board nrf51dk with the SoC nrf51422.

It also uses the new SoC approach, and thereby ensuring that the right
SoC is always used, and cannot accidentially be changed by the user.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

boards/boards_legacy/arm/nrf51dk_nrf51422/Kconfig.board

@@ -1,8 +0,0 @@
-# nRF51 DK NRF51422 board configuration
-
-# Copyright (c) 2016 Linaro Limited.
-# SPDX-License-Identifier: Apache-2.0
-
-config BOARD_NRF51DK_NRF51422
-	bool "nRF51 DK NRF51422"
-	depends on SOC_NRF51822_QFAC

boards/nordic_nrf/nrf51dk/Kconfig.defconfig

@@ -5,9 +5,6 @@
 
 if BOARD_NRF51DK_NRF51422
 
-config BOARD
-	default "nrf51dk_nrf51422"
-
 config BT_CTLR
 	default BT
 

boards/nordic_nrf/nrf51dk/Kconfig.nrf51dk

@@ -0,0 +1,8 @@
+# Copyright (c) 2023 Nordic Semiconductor ASA
+
+# SPDX-License-Identifier: Apache-2.0
+
+config BOARD_NRF51DK
+	bool
+	default y
+	select SOC_NRF51822_QFAC

boards/nordic_nrf/nrf51dk/board.yml

@@ -0,0 +1,5 @@
+board:
+  name: nrf51dk
+  vendor: Nordic Semiconductor
+  socs:
+  - name: nrf51422

boards/nordic_nrf/nrf51dk/doc/index.rst

@@ -148,7 +148,7 @@ the board are working properly with Zephyr:
 
 You can build and flash the examples to make sure Zephyr is running correctly on
 your board. The button and LED definitions can be found in
-:zephyr_file:`boards/arm/nrf51dk_nrf51422/nrf51dk_nrf51422.dts`.
+:zephyr_file:`boards/nordic_nrf/nrf51dk/nrf51dk_nrf51422.dts`.
 
 References
 **********

boards/nordic_nrf/nrf51dk/nrf51dk_nrf51422.yaml

@@ -1,4 +1,4 @@
-identifier: nrf51dk_nrf51422
+identifier: nrf51dk/nrf51422
 name: nRF51-DK-NRF51422
 type: mcu
 arch: arm

boards/nordic_nrf/nrf51dk/nrf51dk_nrf51422_defconfig

@@ -1,9 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 
-CONFIG_SOC_SERIES_NRF51X=y
-CONFIG_SOC_NRF51822_QFAC=y
-CONFIG_BOARD_NRF51DK_NRF51422=y
-
 # enable GPIO
 CONFIG_GPIO=y