ARM: omap2: restore OMAP4 barrier behaviour

Restore the OMAP4 barrier behaviour using the new implementation which
allows multiplatform systems to hook into the mb() and wmb() ARM
implementations to perform any necessary additional barrier maintanence.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Richard Woodruff <r-woodruff2@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>