XXX: ARM: OMAP: Set -Werror flags for arch/arm/*omap*/Makefile

With the remaining build warnings finally gone, let's keep
things that way. Please use this patch to test build new
patches so the new code won't add warnings.

We probably don't want to enable this by default as the
warnings may depend on compiler version.

Signed-off-by: Tony Lindgren <tony@atomide.com>