phy: mt76x8-usb-phy: add slew rate calibration and remove non-mt7628 part

This patch adds slew rate calibration for mt76x8-usb-phy, removes code
which belongs to mt7620, and gets rid of using syscon and regmap by using
clock driver and reset controller.

diff --git a/drivers/phy/Kconfig b/drivers/phy/Kconfig
index e317373..a72f34f 100644
--- a/drivers/phy/Kconfig
+++ b/drivers/phy/Kconfig
@@ -200,6 +200,7 @@
 config MT76X8_USB_PHY
 	bool "MediaTek MT76x8 (7628/88) USB PHY support"
 	depends on PHY
+	depends on SOC_MT7628
 	help
           Support the USB PHY in MT76x8 SoCs