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/openpiton_riscv64_spl_defconfig b/configs/openpiton_riscv64_spl_defconfig
index e041de4..54c59a3 100644
--- a/configs/openpiton_riscv64_spl_defconfig
+++ b/configs/openpiton_riscv64_spl_defconfig
@@ -23,6 +23,7 @@
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="fdt addr ${fdtcontroladdr}; fdt move ${fdtcontroladdr} ${fdt_addr_r}; load mmc ${mmcdev}:${mmcpart} ${kernel_addr_r} ${image}; booti ${kernel_addr_r} - ${fdt_addr_r}; "
 CONFIG_SPL_MAX_SIZE=0x100000
+CONFIG_SPL_BSS_START_ADDR=0x82000000
 # CONFIG_SPL_LEGACY_IMAGE_FORMAT is not set
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set