zephyr: shim: implement a stub signal_is_gpio
zephyr: shim: implement a stub signal_is_gpio

Implement a signal_is_gpio to have the system compile with
CONFIG_ASSERT=y. The function is used to check if the line is SoC
GPIO, but there's no platform with that setup right now, so just
returning true for the moment.

BRANCH=none
BUG=none
TEST=build and run on volteer with asserts enabled

Signed-off-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>
Change-Id: I41d3c11d0915ce5b6e888d7152fb3c3bdc43d054
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3017986
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Commit-Queue: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>

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