phy: phy-mt65xx-usb3: add mediatek directory and rename file

The driver is actually for T-PHY which supports USB3.0, PCIe and SATA,
and supports more SoCs now, but not just only for series of mt65xx SoCs,
so the name of file, data struct, functions etc with 'mt65xx' may cause
misunderstanding when new SoCs are supported. Here rename them to reflect
the real functions and also enhance readability.

And also update MAINTAINERS file to reflect the correct driver

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>