usb-phy-qusb: powerdown PHY during disconnect to avoid leakage

Currently PHY driver is using TCSR_QUSB2PHY_CLAMP_DIG_N_1P8 register
to avoid leakage issue seen during disconnect. But this could fail
USB ports working independently if this register is used as that
could clamp both inputs going to two QUSB2 PHY on platforms having
USB2 port. To fix this, it is recommended to powerdown QUSB2 PHY
by setting powerdown bit in  QUSB2PHY_PORT_POWER_DOWN register
during disconnect and removing writing 0x0 to TCSR register. Removed
updating TCSR register from dts file as separate change.

Change-Id: I6b0915faa346e5d88310106eeddb3c7a1bfb1d08
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>