Convert CONFIG_SPL_STACK to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_STACK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/tinker-rk3288_defconfig b/configs/tinker-rk3288_defconfig
index 32afbeb..ce12b79 100644
--- a/configs/tinker-rk3288_defconfig
+++ b/configs/tinker-rk3288_defconfig
@@ -25,6 +25,8 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SPL_PAD_TO=0x7f8000
 CONFIG_SPL_NO_BSS_LIMIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xff718000
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x2000
 CONFIG_SPL_I2C=y