drivers: flash: Add RDP (readout protection) support for STM32G4x flash

Add support (and tests) for flash readout protection on the STM32G4x series
(cherry picked from commit 2f43b0b)

Original-Signed-off-by: Adam Mitchell <adam.mitchell@brillpower.com>
GitOrigin-RevId: 2f43b0b
Change-Id: I48ac17c13a1b460eb6ec398542311b440d7d1373
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/4984300
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>
Commit-Queue: Al Semjonovs <asemjonovs@google.com>
Tested-by: Al Semjonovs <asemjonovs@google.com>

tests/drivers/flash/stm32/testcase.yaml

@@ -20,3 +20,10 @@ tests:
       - CONFIG_FLASH_STM32_READOUT_PROTECTION=y
     filter: dt_compat_enabled("st,stm32l4-flash-controller") and
             dt_label_with_parent_compat_enabled("storage_partition", "fixed-partitions")
+  drivers.flash.stm32.g4:
+    platform_allow:
+      - nucleo_g474re
+    extra_configs:
+      - CONFIG_FLASH_STM32_READOUT_PROTECTION=y
+    filter: dt_compat_enabled("st,stm32g4-flash-controller") and
+      dt_label_with_parent_compat_enabled("storage_partition", "fixed-partitions")