RISC-V: defconfig: enable clocks, serial console

Enable PRCI clock driver and serial console by default, so the default
upstream defconfig is bootable to a serial console.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>