mmc: add a Kconfig option to enable the support for MMC write operations

This allows using CONFIG_IS_ENABLED(MMC_WRITE) to compile out code
needed only if write support is required.
The option is added for u-boot and for SPL

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/drivers/mmc/mmc_private.h b/drivers/mmc/mmc_private.h
index 1290eed..a9be4b0 100644
--- a/drivers/mmc/mmc_private.h
+++ b/drivers/mmc/mmc_private.h
@@ -28,7 +28,7 @@
 		void *dst);
 #endif
 
-#if !(defined(CONFIG_SPL_BUILD) && !defined(CONFIG_SPL_SAVEENV))
+#if CONFIG_IS_ENABLED(MMC_WRITE)
 
 #if CONFIG_IS_ENABLED(BLK)
 ulong mmc_bwrite(struct udevice *dev, lbaint_t start, lbaint_t blkcnt,
@@ -40,7 +40,7 @@
 ulong mmc_berase(struct blk_desc *block_dev, lbaint_t start, lbaint_t blkcnt);
 #endif
 
-#else /* CONFIG_SPL_BUILD and CONFIG_SPL_SAVEENV is not defined */
+#else /* CONFIG_SPL_MMC_WRITE is not defined */
 
 /* declare dummies to reduce code size. */