arm: mediatek: retrieve ram_base from dts node for armv8 platform

arm: mediatek: retrieve ram_base from dts node for armv8 platform

Now we use fdtdec_setup_mem_size_base() to get DRAM base from fdt ram node
and update gd->ram_base. CFG_SYS_SDRAM_BASE is unused and will be removed.

Also, since mt7622 always passes fdt to linux kernel, there's no need to
assign value to gd->bd->bi_boot_params.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>