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/axm_defconfig b/configs/axm_defconfig
index 1d49c19..2a5458b 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -40,6 +40,8 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x600
 # CONFIG_SPL_LEGACY_IMAGE_FORMAT is not set
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x304000
 CONFIG_SPL_CRC32=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_NAND_RAW_ONLY=y