riscv: sifive_u: Fix clock-names property for ethernet node

The correct property name is clock-names, not clocks-names.

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

macb 100900fc.ethernet: failed to get macb_clk (-2)
macb: probe of 100900fc.ethernet failed with error -2

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>