mailbox: PCC: Move the MAX_PCC_SUBSPACES definition to header file

Move the MAX_PCC_SUBSPACES definition to acpi/pcc.h file in
preparation to add subspace ID support for cppc_acpi driver.

Signed-off-by: George Cherian <george.cherian@cavium.com>
Reviewed-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>