riscv: allow riscv timer to be instantiated via device tree

riscv: allow riscv timer to be instantiated via device tree

For the architectural timer on riscv, there already is a defined
device tree binding[1]. Allow timer instances to be created from
device tree matches, but for now retain the old mechanism, which
registers the timer biggy-back with the CPU.

[1] linux/Documentation/devicetree/bindings/timer/riscv,timer.yaml

Signed-off-by: Torsten Duwe <duwe@suse.de>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>