soc: sifive: shunt ccache driver to drivers/cache

Move the ccache driver over to drivers/cache, out of the drivers/soc
dumping ground, to this new collection point for cache controller
drivers.

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