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/stm32mp15_dhcom_basic_defconfig b/configs/stm32mp15_dhcom_basic_defconfig
index 16dcc4a..fa3cff0 100644
--- a/configs/stm32mp15_dhcom_basic_defconfig
+++ b/configs/stm32mp15_dhcom_basic_defconfig
@@ -30,6 +30,8 @@
 CONFIG_SPL_FOOTPRINT_LIMIT=y
 CONFIG_SPL_MAX_FOOTPRINT=0x3db00
 CONFIG_SPL_LEGACY_IMAGE_FORMAT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x30000000
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_PARTITION=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION=3
 CONFIG_SPL_ENV_SUPPORT=y