Makefile: Move drivers/dma/ into drivers/Makefile

This rule should not be in the top-level Makefile. Move it, making use
of the new LEGACY_DMA Kconfig.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
index ff5b834..bbeec79 100644
--- a/drivers/dma/Kconfig
+++ b/drivers/dma/Kconfig
@@ -56,6 +56,7 @@
 config APBH_DMA
 	bool "Support APBH DMA"
 	depends on MX23 || MX28 || MX6 || MX7 || IMX8 || IMX8M
+	select DMA_LEGACY
 	help
 	  Enable APBH DMA driver.