rockchip: set Pre-reloc malloc pool size to 4kb for rk3368 based boards

The default 1kb pre-reloc malloc pool is not enough for dm
core to enable the dm-pre-reloc device drivers.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>