riscv: sifive_u: Remove handcrafted clock nodes for UART and ethernet

riscv: sifive_u: Remove handcrafted clock nodes for UART and ethernet

In the past we did not have a model for PRCI, hence two handcrafted
clock nodes ("/soc/ethclk" and "/soc/uartclk") were created for the
purpose of supplying hard-coded clock frequencies. But now since we
have added the PRCI support in QEMU, we don't need them any more.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>