spl: Introduce SPL_DM_SPI Kconfig define

This define indicates if DM_SPI shall be supported in SPL. This allows
proper operation of DM converted SPI drivers in SPL, which use
#if !CONFIG_IS_ENABLED(DM_SPI) to also support not yet DM/DTS converted
boards.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 7862a01..562eafe 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -697,6 +697,13 @@
 	  Enable support for loading payloads from UBI. See
 	  README.ubispl for more info.
 
+if SPL_DM
+config SPL_DM_SPI
+	bool "Support SPI DM drivers in SPL"
+	help
+	  Enable support for SPI DM drivers in SPL.
+
+endif
 if SPL_UBI
 config SPL_UBI_LOAD_BY_VOLNAME
 	bool "Support loading volumes by name"