phy: Add generic_{setup,shutdown}_phy() helpers

In drivers usb/host/{ehci,ohci}-generic.c, {ehci,ohci}_setup_phy() and
{ehci,ohci}_shutdown_phy() shares 95% of common code.
Factorize this code in new generic_{setup,shudown}_phy() functions.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>