rockchip: rk356x: use DRAM banks from ATAGS

rockchip: rk356x: use DRAM banks from ATAGS

RK356x-based devices now support creating DRAM banks with proper holes
by reading the ATAGS from Rockchip TPL blob, so let's use that mechanism
instead.

The CONFIG_NR_DRAM_BANK now defaults to 10 which is a safe bet for
reading banks from ATAGS, so let's use the default value instead.

Co-developed-by: Chris Morgan <macromorgan@hotmail.com>
Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>