usb: phy: Don't program REXT if it is not present

On some platform, external REXT is not present. Hence add
optional qcom,no-rext-present property to avoid selecting it
using USB_PHY_RUTNE_SEL register from phy driver.

Change-Id: I1c334d52dd85044dfcaf22473608b58074bcac2e
Signed-off-by: Mayank Rana <mrana@codeaurora.org>