phy: allwinner: phy-sun4i-usb: Add log when probing

When phy-sun4i-usb's probing fails, it does not print the reason in
kernel log, forcing the developer to edit this driver to add info logs.
This commit makes the kernel print the reason of phy-sun4i-usb's probing
failure or a success message.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Tested-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>