rockchip: Clarify docs on SPI writing

We use every second block when creating a SPI image, so update the text to
say this explicitly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>