lib: sbi_pmu: Implement sbi_pmu_counter_fw_read_hi

To support 64 bit firmware counters on RV32 systems, we implement
sbi_pmu_counter_fw_read_hi() which returns the upper 32 bits of
the firmware counter value. On RV64 (or higher) systems, this
function will always return zero.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>