rockchip: mkimage: simplify start/size calculation for rc4_encode

The RC4 encoding works on full blocks, but the calculation of the
starting offset and size are needlessly complicated by using a
reference value known to be offset into a block by the size of the
header and then correcting for the (hard-coded) size of the header
(i.e. 4 bytes).

We change this over to use the RK_SPL_HDR_START directly (which is
known to be on a block boundary).

X-AffectedPlatforms: RK3399-Q7
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>