lib: pmu: support the event ID encoded by a bitmap.

RISC-V privilege specification does not specify how to encode the event ID.
Therefore, each platform is allowed to customize its own encoding rule.
The common encoding methods are as follow, directly assigning a number to an
event, or every bit in the mphmevent CSR controls one specified event or
mixes the above two methods.

To enable OpenSBI to support the above three encoding methods simultaneously,
this patch repurpose the dt property "riscv,raw-event-to-mhpmcounters". The
"riscv,raw-event-to-mhpmcounters" will describes the one or multiple raw
events that could be counted by a set of counters. But, the column number
of "riscv,raw-event-to-mhpmcounters" is extended from 2 to 3. The 1st column
(64bit) is the ID of the raw events. The 2nd column (64bit) represents a
select_mask now to represent the bits used for event ID encoding.
If a platform directly encodes each raw PMU event as a unique ID,
the value of select_mask will be 0xffffffff_ffffffff.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Atish <Patra&lt;atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>