blk: Allow control of the block cache in TPL

Generally block devices are not enabled in TPL, but in case they are,
add a Kconfig option for the block cache. This allows the setting (default
off) to be found with CONFIG_IS_ENABLED().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 33f4aa2..1e6dad8 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -56,6 +56,13 @@
 	help
 	  This option enables the disk-block cache in SPL
 
+config TPL_BLOCK_CACHE
+	bool "Use block device cache in TPL"
+	depends on TPL_BLK
+	default n
+	help
+	  This option enables the disk-block cache in TPL
+
 config IDE
 	bool "Support IDE controllers"
 	select HAVE_BLOCK_DEVICE