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/rock_defconfig b/configs/rock_defconfig
index dfc9257..e99272e 100644
--- a/configs/rock_defconfig
+++ b/configs/rock_defconfig
@@ -27,6 +27,8 @@
 CONFIG_SPL_MAX_SIZE=0x7800
 CONFIG_SPL_PAD_TO=0x7f8000
 CONFIG_SPL_NO_BSS_LIMIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x10087fff
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x2000
 CONFIG_RANDOM_UUID=y