perf/riscv-sbi: Add platform specific firmware event handling

The SBI v2.0 specification pointed to by the link below reserves the
event code 0xffff for platform specific firmware events. Update the driver
to be able to parse and program such events. The platform specific
firmware events must now be specified in the perf command as below:
perf stat -e rCxxx ...
where bits[63:62] = 0x3 of the event config indicate a platform specific
firmware event and xxx indicate the actual event code which is passed
as the event data.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Link: https://github.com/riscv-non-isa/riscv-sbi-doc/releases/download/v2.0/riscv-sbi.pdf
Link: https://lore.kernel.org/r/20240812051109.6496-1-mchitale@ventanamicro.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>