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/sama5d3xek_mmc_defconfig b/configs/sama5d3xek_mmc_defconfig
index 741ab76..4c7a794 100644
--- a/configs/sama5d3xek_mmc_defconfig
+++ b/configs/sama5d3xek_mmc_defconfig
@@ -38,6 +38,8 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL_MAX_SIZE=0x18000
 CONFIG_SPL_BSS_MAX_SIZE=0x80000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x318000
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276