ARM: OMAP2+: separate out OMAP4 restart

Separate out OMAP4 restart and have it similar to other platforms, in
a different file. Main motive is to reuse omap4-common on platforms
other than OMAP4, like AM43x, even if OMAP4 is deselected (otherwise
would have caused build breakage).

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>