drivers: serial_sifive: Skip baudrate config if no input clock

drivers: serial_sifive: Skip baudrate config if no input clock

It is possible that input clock is not available because clk
device was not available and 'clock-frequency' DT property is
also not available.

In this case, instead of failing we should just skip baudrate
config by returning zero.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alexander Graf <agraf@suse.de>