riscv: sivive_u: Add dummy serial clock and aliases entry for uart

The riscv uart needs valid clocks. This requires a refereence
to the clock node. Since the SOC clock is not emulated by qemu,
add a reference to a fixed clock instead. The clock-frequency
entry in the uart node does not seem to be necessary, so drop it.

In addition to a reference to the clock, the driver also needs
an aliases entry for the serial node. Add it as well.

Without this patch, the serial driver fails to instantiate with
the following error message.

sifive-serial 10013000.uart: unable to find controller clock
sifive-serial: probe of 10013000.uart failed with error -2

when trying to boot Linux.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>