configs: rk3288: Increase bootm length

Increase bootm length to 64MB satisfy max gunzip
size, even other rockchip and know SoC are following
same length check.

Reported-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>