perf/cxlpmu: Fix typos in cxl_pmu.c comments and documentation

perf/cxlpmu: Fix typos in cxl_pmu.c comments and documentation

Fix several minor typo errors in comments:
- Remove duplicated word "a" in "a a VID / GroupID".
- Correct "Opcopdes" to "Opcodes" in CXL spec reference.
- Fix spelling of "implemnted" to "implemented".

Improves code readability and documentation consistency.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://lore.kernel.org/r/20250624194350.109790-4-alok.a.tiwari@oracle.com
Signed-off-by: Will Deacon <will@kernel.org>