configs: tinker: Enable SDRAM, SPL stack

SPL sets up SDRAM while in its board_init_f()
function, it is possible for the stack to move
there before board_init_r() is reached.

So it is required to reserve the stack for SDRAM,
with a proper location and size otherwise any
operations during SPL handoff would leads to failure.

On, this particular context tinker-rk3288 SPL is
failing to launch U-Boot proper on SDRAM due to
lack of stack.

 U-Boot SPL 2019.10-rc3-00297-g5ba8b12543 (Sep 12 2019 - 08:50:36 +0530)
 Trying to boot from MMC1
 spl: mmc init failed with error: -110
 SPL: failed to boot from all boot devices
 ### ERROR ### Please RESET the board ###

So, enable SPL_STACK_R_ADDR and it's related-config
items for SDRAM, SPL stack to setup properly.

Reported-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>