rockchip: sdram: add dram bank with usable memory beyond 4GB

Add a second dram bank of usable memory beyond the blob of space for
peripheral near 4GB. Any memory that exists beyond the 4GB mark is added
to the second bank.

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