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/r8a77980_condor_defconfig b/configs/r8a77980_condor_defconfig
index f05590f..d24204d 100644
--- a/configs/r8a77980_condor_defconfig
+++ b/configs/r8a77980_condor_defconfig
@@ -24,6 +24,8 @@
 # CONFIG_BOARD_EARLY_INIT_F is not set
 CONFIG_SYS_MALLOC_BOOTPARAMS=y
 CONFIG_SPL_BSS_MAX_SIZE=0x1000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0xe6304000
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=2068