arm: mediatek: mt8188: fix gd->ram_top limit

arm: mediatek: mt8188: fix gd->ram_top limit

Fix the implementation of the gd->ram_top limit for mt8188.

The intention of the comment about MMC/DMA is correct, but the
implementation was wrong. gd->mon_len is set to the code size of U-Boot,
so trying to set it to limit gd->ram_top does not make sense.

Instead, there is already a get_effective_memsize() weak function that
we can override to implement the required limit on the usable memory
size. This is used to set gd->ram_top in setup_dest_addr().

The comment about the extra SZ_1M needing to be reserved is not correct
as U-Boot already takes care of this (with the actual size of U-Boot) in
the various board_f functions, so it is removed.

This fixes DMA not working on MMC on mt8188.

Reviewed-by: Julien Stephan <jstephan@baylibre.com>
Tested-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20260319-mtk-init-fix-dram-v1-8-6171ec141f40@baylibre.com
Signed-off-by: David Lechner <dlechner@baylibre.com>