refactor(boards): Move planck rev6 to HWMv2.

Move Planck board definition to HWMv2, including versioning tweaks.

app/boards/olkb/planck/Kconfig.planck

@@ -3,6 +3,6 @@
 # Copyright (c) 2020 The ZMK Contributors
 # SPDX-License-Identifier: MIT
 
-config BOARD_PLANCK_REV6
-    bool "Planck V6 Keyboard"
-    depends on SOC_STM32F303XC
+config BOARD_PLANCK
+    bool "Planck Keyboard"
+    select SOC_STM32F303XC

app/boards/olkb/planck/board.yml

@@ -0,0 +1,10 @@
+board:
+  name: planck
+  vendor: olkb
+  socs:
+    - name: stm32f303xc
+  revision:
+    format: major.minor.patch
+    default: 6.0.0
+    revisions:
+      - name: 6.0.0

app/boards/olkb/planck/planck.zmk.yml

@@ -1,6 +1,6 @@
 file_format: "1"
-id: planck_rev6
-name: Planck Rev6
+id: planck
+name: Planck (Rev6)
 type: board
 arch: arm
 features:
@@ -9,3 +9,6 @@ features:
 outputs:
   - usb
 url: https://olkb.com/collections/planck
+revisions:
+  - "6.0.0"
+default_revision: "6.0.0"

app/boards/olkb/planck/planck_6_0_0_defconfig

@@ -1,7 +1,5 @@
 # SPDX-License-Identifier: MIT
 
-CONFIG_SOC_SERIES_STM32F3X=y
-CONFIG_SOC_STM32F303XC=y
 # 72MHz system clock
 CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC=72000000