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/lion-rk3368_defconfig b/configs/lion-rk3368_defconfig
index c03eeca..c700115 100644
--- a/configs/lion-rk3368_defconfig
+++ b/configs/lion-rk3368_defconfig
@@ -38,6 +38,8 @@
 CONFIG_SPL_BOOTROM_SUPPORT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
 CONFIG_TPL_SYS_MALLOC_SIMPLE=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x188000
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x4000
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x200