samples: bluetooth: hci_spi: use pinctrl for nrf board overlay

nRF boards now require usage of pinctrl, migrate them.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/samples/bluetooth/hci_spi/boards/nrf51dk_nrf51422.overlay b/samples/bluetooth/hci_spi/boards/nrf51dk_nrf51422.overlay
index 1517aba..d818e20 100644
--- a/samples/bluetooth/hci_spi/boards/nrf51dk_nrf51422.overlay
+++ b/samples/bluetooth/hci_spi/boards/nrf51dk_nrf51422.overlay
@@ -4,14 +4,24 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+&pinctrl {
+	spi1_default_alt: spi1_default_alt {
+		group1 {
+			psels = <NRF_PSEL(SPIS_SCK, 0, 6)>,
+				<NRF_PSEL(SPIS_MOSI, 0, 5)>,
+				<NRF_PSEL(SPIS_MISO, 0, 4)>,
+				<NRF_PSEL(SPIS_CSN, 0, 3)>;
+		};
+	};
+};
+
 &spi1 {
 	compatible = "nordic,nrf-spis";
 	status = "okay";
-	sck-pin = <6>;
-	mosi-pin = <5>;
-	miso-pin = <4>;
-	csn-pin = <3>;
 	def-char = <0x00>;
+	pinctrl-0 = <&spi1_default_alt>;
+	/delete-property/ pinctrl-1;
+	pinctrl-names = "default";
 
 	bt-hci@0 {
 		compatible = "zephyr,bt-hci-spi-slave";