spl: dm: use CONFIG_IS_ENABLED to test for the DM option

Even though there's now a TPL_DM configuration option, the spl logic
still checks for SPL_DM and thus does not pick up the proper config
option.

This introduces the use of CONFIG_IS_ENABLED(DM) in spl.c to always
pick up the desired configuration option instead of having a
hard-coded check for the SPL variant.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
index 486fb94..fbfc718 100644
--- a/include/linux/kconfig.h
+++ b/include/linux/kconfig.h
@@ -51,12 +51,25 @@
 #define _IS_SPL 1
 #endif
 
+#ifdef CONFIG_TPL_BUILD
+#define _IS_TPL 1
+#endif
+
+#if defined(CONFIG_TPL_BUILD)
+#define config_val(cfg) _config_val(_IS_TPL, cfg)
+#define _config_val(x, cfg) __config_val(x, cfg)
+#define __config_val(x, cfg) ___config_val(__ARG_PLACEHOLDER_##x, cfg)
+#define ___config_val(arg1_or_junk, cfg)  \
+	____config_val(arg1_or_junk CONFIG_TPL_##cfg, CONFIG_##cfg)
+#define ____config_val(__ignored, val, ...) val
+#else
 #define config_val(cfg) _config_val(_IS_SPL, cfg)
 #define _config_val(x, cfg) __config_val(x, cfg)
 #define __config_val(x, cfg) ___config_val(__ARG_PLACEHOLDER_##x, cfg)
 #define ___config_val(arg1_or_junk, cfg)  \
 	____config_val(arg1_or_junk CONFIG_SPL_##cfg, CONFIG_##cfg)
 #define ____config_val(__ignored, val, ...) val
+#endif
 
 /*
  * CONFIG_VAL(FOO) evaluates to the value of