rockchip: doc: update U-Boot location info

The U-Boot location has been moved to block 16384.
This is 8MB, not 4MB.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>