rockchip: Add TPL_MAX_SIZE for RK3288

Per Kever Yang, 32768 is a reasonable max size for TPL on RK3288.

Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>