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_var_som_defconfig b/configs/imx8mn_var_som_defconfig
index a1f9cee..10ed252 100644
--- a/configs/imx8mn_var_som_defconfig
+++ b/configs/imx8mn_var_som_defconfig
@@ -28,6 +28,8 @@
 CONFIG_DEFAULT_FDT_FILE="freescale/imx8mn-var-som-symphony.dtb"
 CONFIG_ARCH_MISC_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