rockchip: nanopc-t6-rk3588: use DRAM banks from ATAGS

rockchip: nanopc-t6-rk3588: use DRAM banks from ATAGS

RK3588-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.

Since ft_board_setup isn't defined anymore, there's no need for
selecting CONFIG_OF_BOARD_SETUP.

Similarly, because the nanopc-t6-rk3588.c would be empty, it is simply
removed, with the (would-be-empty) Makefile as well.

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>