oprofile, x86: Add support for 6 counters (AMD family 15h)

This patch adds support for up to 6 hardware counters for AMD family
15h cpus. There is a new MSR range for hardware counters beginning at
MSRC001_0200 Performance Event Select (PERF_CTL0).

Signed-off-by: Robert Richter <robert.richter@amd.com>