usb: phy: generic: migrate to gpio_desc

Change internal gpio handling from integer gpios into gpio
descriptors. This change only addresses the internal API and
device-tree/ACPI, while the legacy platform data remains integer space
based.

This change is only build compile tested, and very prone to error. I
leave this comment for now in the commit message so that this patch gets
some testing as I'm pretty sure it's buggy.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>