rockchip: correct the bank0 ram size

The bank0 ram size should be the DRAM size minus reserved size,
the DRAM size may be 1GB, 2GB, 4GB, we can not hard code it.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Added DECLARE_GLOBAL_DATA_PTR for RK3328, RK3368 and RK3399:
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>