acpi/cppc: Add support for CPPC Enable register

To enable CPPC on a processor, the OS should write a value "1" to the
CPPC Enable register. Add support for this register.

Since we have a new variable "enable" in cppc_perf_ctrls, rename it
and the associated functions i.e. cppc_perf_ctrls->cppc_ctrls and
cppc_get_perf()->cppc_get_ctrls().

Signed-off-by: Yazen Ghannam <Yazen.Ghannam@amd.com>
[ carved out into a patch, cleaned up, productized ]
Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>