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/qemu-riscv64_spl_defconfig b/configs/qemu-riscv64_spl_defconfig
index 66d1e56..ce66a8e 100644
--- a/configs/qemu-riscv64_spl_defconfig
+++ b/configs/qemu-riscv64_spl_defconfig
@@ -16,6 +16,7 @@
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_DISPLAY_BOARDINFO=y
 CONFIG_SPL_MAX_SIZE=0x100000
+CONFIG_SPL_BSS_START_ADDR=0x84000000
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_MII is not set