powerpc: Move the testing of CPU_FTR_COHERENT_ICACHE into __flush_icache_range

In function flush_icache_range(), we use cpu_has_feature() to test
the feature bit of CPU_FTR_COHERENT_ICACHE. But this seems not optimal
for two reasons:
 a) For ppc32, the function __flush_icache_range() already do this
    check with the macro END_FTR_SECTION_IFSET.
 b) Compare with the cpu_has_feature(), the method of using macro
    END_FTR_SECTION_IFSET will not introduce any runtime overhead.

[And while at it, add the missing required isync] -- BenH

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>