ec_commands: Add a const for the MCHP MEC IO range
ec_commands: Add a const for the MCHP MEC IO range

Some devices that use the CrOS EC only offer the eight I/O ports
required by the Microchip MEC series embedded controller chip via ACPI
or another mechanism.

While this constant is not used in the ec code itself, we should provide
documentation of this fact.

BUG=none
BRANCH=none
TEST=Built successfully

Signed-off-by: Dustin L. Howett <dustin@howett.net>

Change-Id: Ie99fb029334f1f35197be87185d1e842b7bbf4fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3460548
Reviewed-by: Prashant Malani <50de4bccb2902bfab6d92422f06eff0e3bf14c7b@chromium.org>
Commit-Queue: Tzung-Bi Shih <5e6f7578cd434ae5b4c9ffe5250f7a87a25045d7@chromium.org>
Tested-by: Tzung-Bi Shih <5e6f7578cd434ae5b4c9ffe5250f7a87a25045d7@chromium.org>
Reviewed-by: Tzung-Bi Shih <5e6f7578cd434ae5b4c9ffe5250f7a87a25045d7@chromium.org>
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec