omap4: multi-omap: Allow build to work

The musb support is enabled in omap3 platforms. For omap4 only board
support is available and the driver still isn't supported.
Because of this build with omap3_defconfig used for multi-omap
doesn't work on omap4430 sdp.

This patch avoids usb_musb_init() by adding a cpu check
in the board file.

Thanks to Anand Gadiyar and Mankad Maulik for the suggestion
of patching board file instead of musb driver.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>