ACPI / CPPC: optimized cpc_read and cpc_write

cpc_read and cpc_write are used while holding the pcc_lock spin_lock,
so they need to be as fast as possible. acpi_os_read/write_memory
APIs linearly search through a list for cached mapping which is
quite expensive. Since the PCC subspace is already mapped into
virtual address space during initialization, we can just add the
offset and access the necessary CPPC registers.

This patch + similar changes to PCC driver reduce the time per freq.
transition from around 200us to about 20us for the CPPC cpufreq
driver.

Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Acked-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>