serial: sifive: select by default if SOC_SIFIVE

With the aim of dropping direct selects of drivers from Kconfig.socs,
default the SiFive serial drivers to the value of SOC_SIFIVE.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221005171348.167476-3-conor@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>