rockchip: mkimage: Update init size limit for RK3568

The current init size limit of 76KiB is too big to fit in the 64KiB SRAM
on RK3568, sync init size limit from vendor u-boot to fix this.

Set init size limit to 60KiB (-16KiB) for RK3568.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>