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/imx8mn_evk_defconfig b/configs/imx8mn_evk_defconfig
index beb89a7..ead7f6d 100644
--- a/configs/imx8mn_evk_defconfig
+++ b/configs/imx8mn_evk_defconfig
@@ -33,6 +33,8 @@
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
 CONFIG_SPL_LEGACY_IMAGE_FORMAT=y
 CONFIG_SPL_LEGACY_IMAGE_CRC_CHECK=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x980000
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x300
 CONFIG_SPL_I2C=y