rockchip: mkimage: Update init size limit for RK3328

Latest vendor TPL for RK3328 has grown past the current init size limit
of 28KiB, sync the init size limit from vendor u-boot to fix this.

Set init size limit to 30KiB (+2KiB) for RK3328.

This makes it possible to use latest vendor TPL on RK3328 without
getting a size limit error running the mkimage command.

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