Adder8xx: Fix CFG_MONITOR_LEN

Due to increased space usage, U-Boot can no longer be stored in three sectors.
The current U-Boot use just over three flash sectors (197k), and U-Boot will
become corrupt after saving environment variables. This patch adds another 64k
to CFG_MONITOR_LEN.

Signed-off-by: Frank E. Svendsbøe <frank.svendsboe@gmail.com>