rockchip: sdram: honor CFG_SYS_SDRAM_BASE when defining ram regions

rockchip: sdram: honor CFG_SYS_SDRAM_BASE when defining ram regions

Currently the sdram code for arm64 expects CFG_SYS_SDRAM_BASE to be 0.
The ram being in front and the device-area behind it.

The upcoming RK3576 uses a different layout, with the device area
in front the ram, which then also extends past the 4G mark.

Adapt both the generic zone definitions as well as the ATAG parser
to be usable on devices where CFG_SYS_SDRAM_BASE is not 0.

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