omap: Separate out omap2 FS USB platform init functions

We want to split old FS USB platform init code and stop doing pin multiplexing
under plat-omap. First move 24xx specific init code into omap2_usb[012]_init
functions.

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