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/socfpga_agilex_vab_defconfig b/configs/socfpga_agilex_vab_defconfig
index 85ec909..c0384c2 100644
--- a/configs/socfpga_agilex_vab_defconfig
+++ b/configs/socfpga_agilex_vab_defconfig
@@ -29,6 +29,8 @@
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="run fatscript; run mmcfitload; run mmcfitboot"
 CONFIG_SPL_MAX_SIZE=0x40000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x3ff00000
 CONFIG_SPL_BSS_MAX_SIZE=0x100000
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0xffe3f000