iommu/io-pgtable-arm-v7s: Rename the quirk from MTK_4GB to MTK_EXT

In previous mt2712/mt8173, MediaTek extend the v7s to support 4GB dram.
But in the latest mt8183, We extend it to support the PA up to 34bit.
Then the "MTK_4GB" name is not so fit, This patch only change the quirk
name to "MTK_EXT".

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>