rockchip: mkimage: rkspi: include the header sector in the SPI size calculation

Our earlier change broke the generation of SPI images, by excluding the
2K used for header0 from the size-calculation.

This commit makes sure that these are included before calculating the
required total size (including the padding from the 2K-from-every-4K
conversion).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>