sunxi: define SYS_MONITOR_LEN in Kconfig, not _defconfig

Commit 08574ed3 ("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>