usb: fsl: drop USB_FSL_MPH_DR_OF Kconfig symbol

The USB_FSL_MPH_DR_OF symbol is used to ensure the code that interprets
the DR device node is built whenever one of the two drivers (EHCI or
UDC) for the platform is enabled. However, if CONFIG_USB is disabled
and we only support gadget mode, this causes a Kconfig warning:

warning: (USB_FSL_USB2) selects USB_FSL_MPH_DR_OF which has unmet direct dependencies (USB_SUPPORT && USB)

We can avoid this warning by simply no longer using the symbol,
and making sure we enter the drivers/usb/host/ directory when
the UDC driver is enabled that needs the file, and then we use
Makefile syntax to ensure the file is built-in if needed.

There is currently a dependency on CONFIG_OF, but this is redundant,
as we already know that this is set unconditionally for the platforms
that use this driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <balbi@kernel.org>