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/mk808_defconfig b/configs/mk808_defconfig
index fadea65..7df44fb 100644
--- a/configs/mk808_defconfig
+++ b/configs/mk808_defconfig
@@ -36,6 +36,8 @@
 CONFIG_SPL_MAX_SIZE=0x32000
 CONFIG_SPL_PAD_TO=0x7f8000
 CONFIG_SPL_NO_BSS_LIMIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x1008ffff
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x200000
 CONFIG_SPL_SEPARATE_BSS=y