[U-Boot] [U-Boot, 2/3] rockchip: doc: update U-Boot location info

[Goldschmidt Simon]

>> I just found this commit has calculated the size wrong. 16384 blocks should be 8MB, not 4MB.
> 
> Yes, 8MB is what expected here and even Kever[1] commented the same, what is 4MB here? can you elaborate.

I know. It's only wrong in 'doc/README.rockchip' at line 105 where it says:

"image at block 16384 (i.e. 4MB from the start of the SD card)"


Simon