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/imx8mq_phanbell_defconfig b/configs/imx8mq_phanbell_defconfig
index 8348564..6ef649b 100644
--- a/configs/imx8mq_phanbell_defconfig
+++ b/configs/imx8mq_phanbell_defconfig
@@ -34,6 +34,8 @@
 CONFIG_SPL_MAX_SIZE=0x2b000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 CONFIG_SPL_BOARD_INIT=y
+# CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
+CONFIG_SPL_STACK=0x187ff0
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x300
 CONFIG_SPL_I2C=y