cr50: describe GPIO assignment
cr50: describe GPIO assignment

Just a clean up patch attempting to make it easier to new users to
come up to speed with cr50 use of H1 GPIO subsystem.

BRANCH=none
BUG=none
TEST=none

Change-Id: I539e7629ee94ddef26ae1616dc6eb5c151e9d97e
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/412412
Reviewed-by: Aaron Durbin <af0622ab041e8b0f8e5819838dcc93898196e71f@chromium.org>

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