rockchip: config: evb-rk3288: enable SPL_STACK_R

We need a new STACK in SDRAM instead of SRAM so that the EMMC controller
can work properly. The EMMC/SD controller's master is not able to access
SRAM area, it can only access DRAM area.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>