powerpc/64: Avoid isync in flush_dcache_range()

As per ISA an isync is only needed on instruction cache block
invalidate. Remove the same from dcache invalidate.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200320103242.229223-1-aneesh.kumar@linux.ibm.com