RISC-V: Update CSR and interrupt definitions

RISC-V: Update CSR and interrupt definitions

* Add user-mode CSR defininitions.
* Reorder CSR definitions to match the specification.
* Change H mode interrupt comment to 'reserved'.
* Remove unused X_COP interrupt.
* Add user-mode interrupts.
* Remove erroneous until comments on machine mode interrupts.
* Move together paging mode and page table bit definitions.
* Move together interrupt and exception cause definitions.

Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>