rockchip: sdram: fix dram_init_banksize

rockchip: sdram: fix dram_init_banksize

Currently 2.5 GB is calculated as DRAM size for a 1 GB RK322x board
if CONFIG_SPL_OPTEE is set. This is troublesome when booting a
linux kernel since this size will be injected in FDT of the kernel.

gd->bd->bi_dram[0].start (which is basically CONFIG_SYS_SDRAM_BASE)
must not be taken into consideration for calculation of second bank
size, since this offset is already included in calculation of "top".

After applying this patch 992 MB (1024 MB - 32 MB reserved for
optee-os) is correctly calculated and has also been verified on
2 GB boards.

Signed-off-by: Alex Bee <knaerzche@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>