Convert CONFIG_SYS_MONITOR_BASE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MONITOR_BASE

Note that for how this is re-used on some PowePC platforms, we introduce
CONFIG_SPL_SYS_MONITOR_BASE and CONFIG_TPL_SYS_MONITOR_BASE and use the
CONFIG_VAL macro to get the correct value at build time, in the code.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ae350_rv64_defconfig b/configs/ae350_rv64_defconfig
index c30763a..22c0726 100644
--- a/configs/ae350_rv64_defconfig
+++ b/configs/ae350_rv64_defconfig
@@ -9,6 +9,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SYS_LOAD_ADDR=0x100000
 CONFIG_FIT=y
+CONFIG_SYS_MONITOR_BASE=0x88000000
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="RISC-V # "