rockchip: use common sdram function

Replace the sdram_init() in board init and rockchip_sdram_size() in
sdram driver for all the Rockchip SoCs which enable CONFIG_RAM.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Make dram_init() in rk3036-board.c conditional on CONFIG_RAM:
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>