soc: kconfig: add SOC_DEPRECATED_RELEASE

Adding a Kconfig parameter so that we can indicate an SoC is to be
deprecated, similar to what is being done for BOARD_DEPRECATED_RELEASE.

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>
diff --git a/soc/Kconfig b/soc/Kconfig
index 78f0be7..0dc8dfd 100644
--- a/soc/Kconfig
+++ b/soc/Kconfig
@@ -71,3 +71,11 @@
 	  under include/arch/.
 
 endif # ARC || ARM || X86 || NIOS2 || RISCV
+
+config SOC_DEPRECATED_RELEASE
+	string
+	help
+	  This hidden option is set in the SoC configuration and indicates
+	  the Zephyr release that the SoC configuration will be removed.
+	  When set, any build for that SoC will generate a clearly visible
+	  deprecation warning.