uboot-mediatek: convert env in mtd to upstream implementation

uboot-mediatek: convert env in mtd to upstream implementation

In the upstream implementation of env in mtd, ENV_MTD_NAME symbol
was renamed to ENV_MTD_DEV. Therefore, we have to adjust the symbol
to make it work properly. ENV_SIZE_REDUND symbol will be removed
in this patch because the upstream env in mtd implementation no
longer requires it.

Fixes: 41a9c9de66a7 ("uboot-mediatek: update to v2025.07")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>