cache: sifive_ccache: Optimize cache flushes

Fence instructions are required only at the beginning and the end of
a flush operation, not separately for each cache line being flushed.
Speed up cache flushes by about 15% by removing the extra fences.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>