usb: USB_XHCI_PCI depends on PCI

Compiling with CONFIG_USB_XHCI_PCI and CONFIG_PCI=n results in

    usb/host/xhci-pci.c:48:(.text.xhci_pci_probe+0x44):
    undefined reference to `dm_pci_write_config32

Add the missing Kconfig dependency.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Marek Vasut <marex@denx.de>