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

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>