kbuild: refactor some makefiles

[1] Move driver/core/, driver/input/ and drivers/input/ entries
    from the top Makefile to drivers/Makefile

[2] Remove the conditional by CONFIG_DM in drivers/core/Makefile
    because the whole drivers/core directory is already selected
    by CONFIG_DM in the upper level

[3] Likewise for CONFIG_DM_DEMO in drivers/demo/Makefile

[4] Simplify common/Makefile - both CONFIG_DDR_SPD and
    CONFIG_SPD_EEPROM are boolean macros so they can directly
    select objects

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/drivers/demo/Makefile b/drivers/demo/Makefile
index baaa2ba..171ddf3 100644
--- a/drivers/demo/Makefile
+++ b/drivers/demo/Makefile
@@ -4,6 +4,6 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_DM_DEMO) += demo-uclass.o demo-pdata.o
+obj-y += demo-uclass.o demo-pdata.o
 obj-$(CONFIG_DM_DEMO_SIMPLE) += demo-simple.o
 obj-$(CONFIG_DM_DEMO_SHAPE) += demo-shape.o