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/Kconfig b/drivers/mmc/Kconfig
index d196eed..ab0627a 100644
--- a/drivers/mmc/Kconfig
+++ b/drivers/mmc/Kconfig
@@ -10,6 +10,13 @@
 	  If you want MMC/SD/SDIO support, you should say Y here and
 	  also to your specific host controller driver.
 
+config MMC_WRITE
+	bool "support for MMC/SD write operations"
+	depends on MMC
+	default y
+	help
+	  Enable write access to MMC and SD Cards
+
 config DM_MMC
 	bool "Enable MMC controllers using Driver Model"
 	depends on DM