Convert CONFIG_SPL_BSS_START_ADDR to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_BSS_START_ADDR

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx8mm-cl-iot-gate_defconfig b/configs/imx8mm-cl-iot-gate_defconfig
index 2494d1a..e630bca 100644
--- a/configs/imx8mm-cl-iot-gate_defconfig
+++ b/configs/imx8mm-cl-iot-gate_defconfig
@@ -26,6 +26,8 @@
 CONFIG_OF_SYSTEM_SETUP=y
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SPL_MAX_SIZE=0x25000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x910000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 CONFIG_SPL_BOARD_INIT=y
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set