feat(st-sdmmc2): allow compatible to be defined in platform code

Put DT_SDMMC2_COMPAT under #ifndef. Keep the default value if it is not
defined in platform code.

Change-Id: I611baaf1fc622d33e655ee2c78d9c287baaa6a67
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/drivers/st/mmc/stm32_sdmmc2.c b/drivers/st/mmc/stm32_sdmmc2.c
index 3f709a7..40641b5 100644
--- a/drivers/st/mmc/stm32_sdmmc2.c
+++ b/drivers/st/mmc/stm32_sdmmc2.c
@@ -125,7 +125,9 @@
 #define POWER_OFF_DELAY			2
 #define POWER_ON_DELAY			1
 
+#ifndef DT_SDMMC2_COMPAT
 #define DT_SDMMC2_COMPAT		"st,stm32-sdmmc2"
+#endif
 
 static void stm32_sdmmc2_init(void);
 static int stm32_sdmmc2_send_cmd_req(struct mmc_cmd *cmd);