rockchip: Makefile: allow selective inclusion of sdram_common.o from TPL/SPL/U-Boot

The utility functions in sdram_common.c will be useful both for some
SPL implementations (and if unused, the linked will discard these
anyway) and for the full U-Boot stage.

This changes selects sdram_common.o through the $(SPL_TPL_) macro to
allow better control of its inclusion through the CONFIG_ROM,
CONFIG_SPL_RAM or CONFIG_TPL_RAM options.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/mach-rockchip/Makefile

@@ -5,6 +5,7 @@
 #
 
 obj-$(CONFIG_ROCKCHIP_BROM_HELPER) += bootrom.o
+obj-$(CONFIG_$(SPL_TPL_)RAM) += sdram_common.o
 
 ifdef CONFIG_TPL_BUILD
 obj-$(CONFIG_ROCKCHIP_RK3188) += rk3188-board-tpl.o
@@ -21,9 +22,6 @@ obj-$(CONFIG_ROCKCHIP_RK3188) += rk3188-board.o
 obj-$(CONFIG_ROCKCHIP_RK322X) += rk322x-board.o
 obj-$(CONFIG_ROCKCHIP_RK3288) += rk3288-board.o
 obj-$(CONFIG_ROCKCHIP_RK3036) += rk3036-board.o
-ifdef CONFIG_RAM
-obj-y += sdram_common.o
-endif
 endif
 ifndef CONFIG_ARM64
 obj-y += rk_timer.o