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/sifive_unmatched_defconfig b/configs/sifive_unmatched_defconfig
index 989029b..23f8a4f 100644
--- a/configs/sifive_unmatched_defconfig
+++ b/configs/sifive_unmatched_defconfig
@@ -29,6 +29,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_ID_EEPROM=y
 CONFIG_SPL_MAX_SIZE=0x100000
+CONFIG_SPL_BSS_START_ADDR=0x85000000
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0x81cfe60
 CONFIG_SPL_DM_SPI_FLASH=y