Kconfig: Move BOUNCE_BUFFER under driver options

This option does not belong at the top level. Move it under generic
driver options.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/Kconfig b/drivers/core/Kconfig
index 1ca5d66..07d3a6a 100644
--- a/drivers/core/Kconfig
+++ b/drivers/core/Kconfig
@@ -286,4 +286,15 @@
 	  information such as P states and T stages. Also included is a way
 	  to create a GNVS table and set it up.
 
+config BOUNCE_BUFFER
+	bool "Include bounce buffer API"
+	help
+	  Some peripherals support DMA from a subset of physically
+	  addressable memory only.  To support such peripherals, the
+	  bounce buffer API uses a temporary buffer: it copies data
+	  to/from DMA regions while managing cache operations.
+
+	  A second possible use of bounce buffers is their ability to
+	  provide aligned buffers for DMA operations.
+
 endmenu