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/am65x_evm_a53_defconfig b/configs/am65x_evm_a53_defconfig
index af8160c..d07edc2 100644
--- a/configs/am65x_evm_a53_defconfig
+++ b/configs/am65x_evm_a53_defconfig
@@ -36,6 +36,8 @@
 CONFIG_LOGLEVEL=7
 CONFIG_CONSOLE_MUX=y
 CONFIG_SPL_MAX_SIZE=0x58000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x80a00000
 CONFIG_SPL_BSS_MAX_SIZE=0x80000
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_STACK_R=y