sifive_u: Set 'clock-frequency' DT property for SiFive UART

sifive_u: Set 'clock-frequency' DT property for SiFive UART

The 'clock-frequency' DT property is required by U-Boot to compute
the divider value. This patch sets the 'clock-frequency' DT property
of the SiFive UART device tree node (similar to virt machine).

Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>