ARM: OMAP: sram: fix OMAP4 errata handling

OMAP4-specific code should be executed only if we are running on
OMAP4. Otherwise it may break multi-OMAP kernels. Found by reading
the code.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>