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/common/spl/Kconfig b/common/spl/Kconfig
index 9d35f41..d686b1e 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -301,6 +301,7 @@
 config SPL_SAVEENV
 	bool "Support save environment"
 	depends on SPL_ENV_SUPPORT
+	select SPL_MMC_WRITE if ENV_IS_IN_MMC
 	help
 	  Enable save environment support in SPL after setenv. By default
 	  the saveenv option is not provided in SPL, but some boards need
@@ -415,6 +416,14 @@
 	  this option to build the drivers in drivers/mmc as part of an SPL
 	  build.
 
+config SPL_MMC_WRITE
+	bool "MMC/SD/SDIO card support for write operations in SPL"
+	depends on SPL_MMC_SUPPORT
+	default n
+	help
+	  Enable write access to MMC and SD Cards in SPL
+
+
 config SPL_MPC8XXX_INIT_DDR_SUPPORT
 	bool "Support MPC8XXX DDR init"
 	help