serial/sifive: select SERIAL_EARLYCON

The sifive serial driver implements earlycon support, but unless
another driver is built in that supports earlycon support it won't
be usable.  Explicitly select SERIAL_EARLYCON instead.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Link: https://lore.kernel.org/r/20190910055923.28384-1-hch@lst.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -1032,6 +1032,7 @@ config SERIAL_SIFIVE_CONSOLE
 	bool "Console on SiFive UART"
 	depends on SERIAL_SIFIVE=y
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Select this option if you would like to use a SiFive UART as the
 	  system console.