rockchip: mkimage: Use an existing macro instead of a decimal value

Depending on the SoC, a header of four characters is prepended to the
image. There is already a macro defining the number of characters:
RK_SPL_HDR_SIZE, so use it instead of hardcoding "4".

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>