powerpc/usb: fix bug of CPU hang when missing USB PHY clock
when missing USB PHY clock, u-boot will hang during USB
initialization when issuing "usb start". We should check
USBGP[PHY_CLK_VALID] bit to avoid CPU hanging in this case.
Due to controller issue of PHY_CLK_VALID in ULPI mode, we set
USB_EN before checking PHY_CLK_VALID, otherwise PHY_CLK_VALID
doesn't work.
Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com> Acked-by: Marek Vasut <marex@denx.de>