RISC-V: Use [ms]counteren CSRs when priv ISA >= v1.10

RISC-V: Use [ms]counteren CSRs when priv ISA >= v1.10

Privileged ISA v1.9.1 defines mscounteren and mucounteren:

* mscounteren contains a mask of counters available to S-mode
* mucounteren contains a mask of counters available to U-mode

Privileged ISA v1.10 defines mcounteren and scounteren:

* mcounteren contains a mask of counters available to S-mode
* scounteren contains a mask of counters available to U-mode

mcounteren and scounteren CSR registers were implemented
however they were not honoured for counter accesses when
the privilege ISA was >= v1.10. This fix solves the issue
by coalescing the counter enable registers. In addition
the code now  generates illegal instruction exceptions
for accesses to the counter enabled registers depending
on the privileged ISA version.

- Coalesce mscounteren and mcounteren into one variable
- Coalesce mucounteren and scounteren into one variable
- Makes mcounteren and scounteren CSR accesses generate
  illegal instructions when the privileged ISA <= v1.9.1
- Makes mscounteren and mucounteren CSR accesses generate
  illegal instructions when the privileged ISA >= v1.10

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: Michael Clark <mjc@sifive.com>