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/rock64-rk3328_defconfig b/configs/rock64-rk3328_defconfig
index 7026b6f..aa4cdde 100644
--- a/configs/rock64-rk3328_defconfig
+++ b/configs/rock64-rk3328_defconfig
@@ -33,6 +33,8 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 # 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=0x400000
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_POWER=y