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-uclass.c b/drivers/mmc/mmc-uclass.c
index 793196b..26c6ab7 100644
--- a/drivers/mmc/mmc-uclass.c
+++ b/drivers/mmc/mmc-uclass.c
@@ -370,7 +370,7 @@
 
 static const struct blk_ops mmc_blk_ops = {
 	.read	= mmc_bread,
-#ifndef CONFIG_SPL_BUILD
+#if CONFIG_IS_ENABLED(MMC_WRITE)
 	.write	= mmc_bwrite,
 	.erase	= mmc_berase,
 #endif