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_evk_defconfig b/configs/imx8mn_evk_defconfig
index ead7f6d..a17b839 100644
--- a/configs/imx8mn_evk_defconfig
+++ b/configs/imx8mn_evk_defconfig
@@ -27,6 +27,8 @@
 CONFIG_ARCH_MISC_INIT=y
 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_BOARD_INIT=y
 CONFIG_SPL_BOOTROM_SUPPORT=y