target/riscv: Add sscofpmf extension support

target/riscv: Add sscofpmf extension support

The Sscofpmf ('Ss' for Privileged arch and Supervisor-level extensions,
and 'cofpmf' for Count OverFlow and Privilege Mode Filtering)
extension allows the perf to handle overflow interrupts and filtering
support. This patch provides a framework for programmable
counters to leverage the extension. As the extension doesn't have any
provision for the overflow bit for fixed counters, the fixed events
can also be monitoring using programmable counters. The underlying
counters for cycle and instruction counters are always running. Thus,
a separate timer device is programmed to handle the overflow.

Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220824221701.41932-2-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>