platform/x86: intel_pmc_ipc: Mark ipc_data_readb() as __maybe_unused

The function is currently not used, however it is part of the API and
might be used in the future. Adding the attribute fixes the following
warning when building with clang:

    drivers/platform/x86/intel_pmc_ipc.c:189:18: error: unused function
        'ipc_data_readb' [-Werror,-Wunused-function]

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>