serial: make SiFive serial drivers depend on ARCH_ symbols

As part of converting RISC-V SOC_FOO symbols to ARCH_FOO to match the
use of such symbols on other architectures, convert the SiFive serial
driver Kconfig entries from the SOC_ symbols to ARCH_ instead.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230406-carnival-aspirate-fcf69a30078c@spud
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>