[PATCH] ARM: OMAP: Clean up cpu.h, part 2

Remove old optimizations for 1610; These are no longer
needed as 1610, 5912 and 1710 are consdered a single
group. Also adds a placeholder for cpu_is_omap2420().

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>