sifive_u: Add clock DT node for GEM ethernet

sifive_u: Add clock DT node for GEM ethernet

The GEM ethernet on SiFive unleashed has fixed input clock
of 125MHz as-per SiFive FU540 manual. This patch updates FDT
generation for QEMU sifive_u machine to provide fixed-rate
clock for GEM ethernet.

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>