RISC-V: Improve SBI PMU extension related definitions

RISC-V: Improve SBI PMU extension related definitions

This patch fixes/improve few minor things in SBI PMU extension
definition.

1. Align all the firmware event names.
2. Add macros for bit positions in cache event ID & ops.

The changes were small enough to combine them together instead
of creating 1 liner patches.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>