zephyr: add IOEX_ constants to gpio_signal.h
zephyr: add IOEX_ constants to gpio_signal.h

Multiple headers and platform/ec files need some of the IOEX_*
constants defined, even if we don't support IO expanders (yet).  Add
this as a stopgap.

BUG=b:171312361
BRANCH=none
TEST=compile for volteer

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I78fa80047683eff9dca318a1656c21448a76579f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2548301
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>

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