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/imx8mn_venice_defconfig b/configs/imx8mn_venice_defconfig
index f2ddb98..7479f99 100644
--- a/configs/imx8mn_venice_defconfig
+++ b/configs/imx8mn_venice_defconfig
@@ -33,6 +33,8 @@
 CONFIG_PREBOOT="gsc wd-disable"
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SPL_MAX_SIZE=0x25000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x950000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 CONFIG_SPL_BOOTROM_SUPPORT=y
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set