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_beacon_defconfig b/configs/imx8mn_beacon_defconfig
index 95e4cc4..3495bc0 100644
--- a/configs/imx8mn_beacon_defconfig
+++ b/configs/imx8mn_beacon_defconfig
@@ -32,6 +32,8 @@
 CONFIG_DEFAULT_FDT_FILE="imx8mn-beacon-kit.dtb"
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_MAX_SIZE=0x25000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x95e000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_BOOTROM_SUPPORT=y