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/rockpro64-rk3399_defconfig b/configs/rockpro64-rk3399_defconfig
index 8a66931..fff00fb 100644
--- a/configs/rockpro64-rk3399_defconfig
+++ b/configs/rockpro64-rk3399_defconfig
@@ -25,6 +25,8 @@
 CONFIG_SPL_PAD_TO=0x7f8000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x400000
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x10000
 CONFIG_SPL_SPI_LOAD=y