Use capitals in the names of the board ID symbols and add the PCH_
prefix to each of them for consistency.

Also rename the board_ids enum accordingly.

No intentional functional impact.
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: NDaniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: NZhang Rui <rui.zhang@intel.com>
Reviewed-by: NZhang Rui <rui.zhang@intel.com>