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/nyan-big_defconfig b/configs/nyan-big_defconfig
index 4433b65..1ffddbe 100644
--- a/configs/nyan-big_defconfig
+++ b/configs/nyan-big_defconfig
@@ -25,6 +25,8 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SPL_FOOTPRINT_LIMIT=y
 CONFIG_SPL_MAX_FOOTPRINT=0xef8100
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x800ffffc
 CONFIG_SYS_PROMPT="Tegra124 (Nyan-big) # "
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=2087