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/vexpress_ca9x4_defconfig b/configs/vexpress_ca9x4_defconfig
index 10e93cf..ca7aef6 100644
--- a/configs/vexpress_ca9x4_defconfig
+++ b/configs/vexpress_ca9x4_defconfig
@@ -9,6 +9,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="vexpress-v2p-ca9"
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SYS_LOAD_ADDR=0x90000000
+CONFIG_SYS_MONITOR_BASE=0x40000000
 CONFIG_BOOTCOMMAND="run distro_bootcmd; run bootflash"
 CONFIG_DEFAULT_FDT_FILE="vexpress-v2p-ca9.dtb"
 # CONFIG_DISPLAY_CPUINFO is not set