ARM: davinci: da8xx: add usb phy clocks

Up to this point, the USB phy clock configuration was handled manually in
the board files and in the usb drivers. This adds proper clocks so that
the usb drivers can use clk_get and clk_enable and not have to worry about
the details. Also, the related code is removed from the board files and
replaced with the new clock registration functions.

This also removes the #if IS_ENABLED(CONFIG_USB_MUSB_HDRC) around the musb
declaration and renames the musb platform device so that we can reference
it from the usb20 clock even if the musb device is not used.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>