feat(boards): Add Mikoto board

* Supports selecting from several possible charge currents

Co-authored-by: Pete Johanson <peter@peterjohanson.com>

app/boards/arm/mikoto/mikoto_520.yaml

@@ -0,0 +1,15 @@
+identifier: mikoto_520
+name: mikoto_520
+type: mcu
+arch: arm
+toolchain:
+  - zephyr
+  - gnuarmemb
+  - xtools
+supported:
+  - adc
+  - usb_device
+  - ble
+  - ieee802154
+  - pwm
+  - watchdog