cppc_acpi.c: Introduce new function to get transition latency for FFH

When the DesiredPerformanceRegister is in PCC, the transition latency
is determined by using PCCT. However, if the DesiredPerformanceRegister
is an FFH address, we need a new weak function to return architecture
specific latency value. Add a new weak function for this purpose.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>