rockchip: Fix rkimage format for SPL boot over USB

The 'rkimage' format used for booting rockchip boards over USB seems to
have been broken since commit 7bf274b9caab ("rockchip: mkimage: use
imagename to select spl hdr & spl size"). That commit adds an offset of
RK_SPL_HDR_START(=2048) to the location the 'RKxx' header is written
at. However the bootrom expects this header to be the first four bytes of
the image, not at offset 2048. This appears to have been a copy paste
error since the 'rksd' and 'rkspi' image types do require this offset.

Furthermore commit 111bcc4fb6cb ("rockchip: mkimage: pad the header to
8-bytes (using a 'nop') for RK3399"), commit 3d54eabcafec9 ("rockchip:
spl: RK3399: use boot0 hook to create space for SPL magic") and
commit 308277569229 ("rockchip: mkimage: update rkimage to support
pre-padded payloads") changed the way the space for the 'RKxx' header is
allocated and written to the image without adjusting 'rkimage'.

This commit fixes those mistakes and makes it possible to load u-boot SPL
over USB once more.

(Tested on RK3399)

Signed-off-by: Daniel Gröber <daniel@dps.uibk.ac.at>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>