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/uniphier_ld4_sld8_defconfig b/configs/uniphier_ld4_sld8_defconfig
index 10f00b5..ceb5083 100644
--- a/configs/uniphier_ld4_sld8_defconfig
+++ b/configs/uniphier_ld4_sld8_defconfig
@@ -21,6 +21,8 @@
 CONFIG_SPL_MAX_SIZE=0x10000
 CONFIG_SPL_PAD_TO=0x20000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x100000
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_NOR_SUPPORT=y
 CONFIG_CMD_CONFIG=y