rockchip: rk3399: correct SPL_MAX_SIZE

The SPL_MAX_SIZE is the internal memory size minux the space
used by bootrom.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>