sunxi: define SYS_MONITOR_LEN in Kconfig, not _defconfig

Commit 08574ed339fb ("Convert CONFIG_SYS_MONITOR_LEN to Kconfig") moved
the definition of said config variable from the common sunxi header to
*every board's* defconfig.
This is a platform choice, not board specific, so remove the variable
from there, instead set the one value for all Allwinner boards in
Kconfig.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
diff --git a/Kconfig b/Kconfig
index 67f4646..0cdc965 100644
--- a/Kconfig
+++ b/Kconfig
@@ -586,6 +586,7 @@
 config SYS_MONITOR_LEN
 	int "Maximum size in bytes reserved for U-Boot in memory"
 	default 1048576 if X86
+	default 786432 if ARCH_SUNXI
 	default 0
 	help
 	  Size of memory reserved for monitor code, used to determine