riscv: dts: Re-organize the DT nodes

As per the convention for any SOC device with external connection,
define only device DT node in SOC DTSi file with status = "disabled"
and enable device in Board DTS file with status = "okay"

Reported-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>