rockchip: simplify binman image dependencies addition to INPUTS

By factoring SPL check in the first condition, this makes the checks a
bit less convoluted and more readable.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>