riscv: sifive/fu540: Drop NET_RANDOM_ETHADDR

This option was enabled during the earlier U-Boot porting time. Now
we already have the OTP driver in place and the unique MAC address
is read from the OTP, there is no need to turn on this option.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Leo Liang <ycliang@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Tested-by: Pragnesh Patel <pragnesh.patel@sifive.com>