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/common/Makefile b/common/Makefile
index 66c4f2f..b19d379 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -243,13 +243,8 @@
 #environment
 obj-y += env_common.o
 #others
-ifdef CONFIG_DDR_SPD
-SPD := y
-endif
-ifdef CONFIG_SPD_EEPROM
-SPD := y
-endif
-obj-$(SPD) += ddr_spd.o
+obj-$(CONFIG_DDR_SPD) += ddr_spd.o
+obj-$(CONFIG_SPD_EEPROM) += ddr_spd.o
 obj-$(CONFIG_HWCONFIG) += hwconfig.o
 obj-$(CONFIG_BOUNCE_BUFFER) += bouncebuf.o
 obj-y += console.o