rockchip: px30: default TPL_SYS_MALLOC_F_LEN to 0x600 on PX30 Kconfig level

This is the kind of setting that typically doesn't need to be changed
between boards based on the same SoC, so let's make it the default in
PX30 Kconfig so we don't have to care about it in the defconfig if we
don't want to.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>