Clean up usage of icache_disable/dcache_disable

There is no point in disabling the icache on 7xx/74xx/86xx parts and not
also flushing the icache.  All callers of invalidate_l1_instruction_cache()
call icache_disable() right after.  Make it so icache_disable() calls
invalidate_l1_instruction_cache() for us.

Also, dcache_disable() already calls dcache_flush() so there is no point
in the explicit calls of dcache_flush().

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>