usb: phy: introduce ->set_vbus() method

this method will be used to enable or disable
the charge pump.

Whenever we have DRD devices, we need to be
able to turn VBUS on or off whenever we want.

Note that in the ideal case, this would be
controlled by the ID-pin Interrupt, but not
all devices have ID-pin properly routed since
manufacturers can choose to save that trace
if they're building a host-only product out
of a DRD IP.

This is also useful during debugging where
we might not have the proper cable hanging
around.

Signed-off-by: Felipe Balbi <balbi@ti.com>