arm: mediatek: mt8189: fix gd->bd->bi_dram[0].size

Use board_get_usable_ram_top() instead of get_effective_memsize() to
set gd->ram_top to something <= 4GiB.

Both board_get_usable_ram_top() and get_effective_memsize() are used to
set gd->ram_top in setup_dest_addr(). However, get_effective_memsize()
also sets gd->bd->bi_dram[0].size in dram_init_banksize(), which is
undesirable.

We can just use board_get_usable_ram_top() now to set gd->ram_top to the
correct value instead. This is possible now, thanks to
LMB_LIMIT_DMA_BELOW_RAM_TOP being set by default for 64-bit Mediatek
ARM SoCs.

Fixes: a7c682565b4b ("arm: mediatek: add support of MT8189 SoC family")
Link: https://patch.msgid.link/20260615-mtk-fix-ram-size-v2-6-f72cfc52ce58@baylibre.com
Signed-off-by: David Lechner <dlechner@baylibre.com>