Convert CONFIG_CMD_MAX6957 to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_MAX6957

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/board/work-microwave/work_92105/Kconfig b/board/work-microwave/work_92105/Kconfig
index 1fde4b2..4bc34ed 100644
--- a/board/work-microwave/work_92105/Kconfig
+++ b/board/work-microwave/work_92105/Kconfig
@@ -17,4 +17,11 @@
 	help
 	  This controls the LCD driver.
 
+config CMD_MAX6957
+	bool "Enable 'max6957aax' PMIC command"
+	help
+	  DEPRECATED: Needs conversion to driver model.
+
+	  This allows PMIC registers to be read and written.
+
 endif