ARM: OMAP: Add write memory barriers to OMAP2 clock code

After adjusting clock parameters, OMAP2 CPUs need a memory
barrier to make sure the changes go into effect immediately.
Otherwise bad things will happen if we try to access the
peripheral whose clock is just being enabled.

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>