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/Makefile b/drivers/Makefile
index b22b109..d8361d9 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -1,3 +1,5 @@
+obj-$(CONFIG_DM) += core/
+obj-$(CONFIG_DM_DEMO) += demo/
 obj-$(CONFIG_BIOSEMU) += bios_emulator/
 obj-y += block/
 obj-$(CONFIG_BOOTCOUNT_LIMIT) += bootcount/
@@ -16,3 +18,4 @@
 obj-$(CONFIG_QE) += qe/
 obj-y += memory/
 obj-y += pwm/
+obj-y += input/