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-microgea-stm32mp1-microdev2_defconfig b/configs/stm32mp15-microgea-stm32mp1-microdev2_defconfig
index 39c2f29..86d5547 100644
--- a/configs/stm32mp15-microgea-stm32mp1-microdev2_defconfig
+++ b/configs/stm32mp15-microgea-stm32mp1-microdev2_defconfig
@@ -19,6 +19,8 @@
 CONFIG_BOOTCOMMAND="run bootcmd_stm32mp"
 CONFIG_SPL_FOOTPRINT_LIMIT=y
 CONFIG_SPL_MAX_FOOTPRINT=0x3db00
+# 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