omap: Move omap2 FS USB platform init code into mach-omap2/usb-fs.c

Move omap2 FS USB platform init code into mach-omap2/usb-fs.c. This will
allow further work later on to use omap hwmod for initializing the
device.

Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>