refactor(boards): Switch proton_c to newer pinmux approach.

See: zephyrproject-rtos/zephyr#28999
PR: zmkfirmware#736

Author: petejohanson

Diff for: app/boards/arm/proton_c/CMakeLists.txt

@@ -48,5 +48,5 @@
 };
 
 pro_micro_i2c: &i2c1 {};
-pro_micro_spi: &spi1 {};
+pro_micro_spi: &spi2 {};
 pro_micro_serial: &usart1 {};

Diff for: app/boards/arm/proton_c/arduino_pro_micro_pins.dtsi

@@ -6,6 +6,7 @@
 
 /dts-v1/;
 #include <st/f3/stm32f303Xc.dtsi>
+#include <st/f3/stm32f303c(b-c)tx-pinctrl.dtsi>
 #include "arduino_pro_micro_pins.dtsi"
 
 / {
@@ -26,6 +27,18 @@
 	};
 };
 
+&usart1 {
+	pinctrl-0 = <&usart1_tx_pa9 &usart1_rx_pa10>;
+};
+
+&spi2 {
+	pinctrl-0 = <&spi2_sck_pb13 &spi2_miso_pb14 &spi2_mosi_pb15>;
+};
+
+&i2c1 {
+	pinctrl-0 = <&i2c1_scl_pb6 &i2c1_sda_pb7>;
+};
+
 &usb {
 	status = "okay";
 };