mtd: mtk-nor: modify functions' name more generally

Since more and more Mediatek's SoC can use this driver to
control spi-nor flash, functions' name with "mt8173_" is
no longer properly. Replacing "mt8173_" with "mtk_" will
be more accurate to describe these functions' usable scope.
Signed-off-by: Guochun Mao <guochun.mao@mediatek.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>