ACPI / PPTT: fix build when CONFIG_ACPI_PPTT is not enabled

Though CONFIG_ACPI_PPTT is selected by platforms and nor user visible,
it may be useful to support the build with CONFIG_ACPI_PPTT disabled.

This patch adds the missing dummy/boiler plate implementation to fix
the build.

Acked-by: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>