usb: musb: AM35x: moving internal phy functions out of usb_musb.c file

Moved all the board specific internal PHY functions out of usb_musb.c file
as this file is shared between the OMAP2+ and AM35xx platforms.
There exists a file which has the functions specific to internal PHY
used for OMAP4 platform. Moved all phy specific functions to this file
and passing these functions through board data in the board file.

Signed-off-by: Hema HK <hemahk@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>