rk3399: Add boot flash script offset, size

Most of the SPI flash devices in rockchip (rk3399)
are 16MiB size. So, let's use the script offset at
the end of 8K. 

This way it cannot overlap any offsets being used
by software components in flash layout.

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