devres: make Devres optional with CONFIG_DEVRES

Currently, Devres requires additional 16 byte for each allocation,
which is not so insignificant in some cases.

Add CONFIG_DEVRES to make this framework optional.
If the option is disabled, devres functions fall back to
non-managed variants.  For example, devres_alloc() to kzalloc(),
devm_kmalloc() to kmalloc(), etc.

Because devres_head is also surrounded by an ifdef conditional,
there is no memory overhead when CONFIG_DEVRES is disabled.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/Makefile b/drivers/core/Makefile
index 260833e..ccc2fd4 100644
--- a/drivers/core/Makefile
+++ b/drivers/core/Makefile
@@ -4,7 +4,8 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y	+= device.o lists.o root.o uclass.o util.o devres.o
+obj-y	+= device.o lists.o root.o uclass.o util.o
+obj-$(CONFIG_DEVRES) += devres.o
 ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_OF_CONTROL) += simple-bus.o
 endif