zephyr: move gpio_commands.c to CONFIG_PLATFORM_EC list
zephyr: move gpio_commands.c to CONFIG_PLATFORM_EC list

Adding ${PLATFORM_EC}/common/gpio_commands.c is the only file in this
CMakeLists.txt which can get compiled without CONFIG_PLATFORM_EC=y.

This fixes it so that that including the platform/ec module in the
build should do nothing unless CONFIG_PLATFORM_EC=y, which is
important for non-EC projects (i.e., chameleon).

BUG=b:180545676
BRANCH=none
TEST=zmake testall
TEST=build chameleon with zmake (requires follow-on CLs)

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I604378b969f143e7b3f20a7858470330e093244f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2702318
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

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