zephyr: Add CONFIG_EXTPOWER_GPIO to the shim
zephyr: Add CONFIG_EXTPOWER_GPIO to the shim

This is not strictly necessary, but shows up as a delta from the
existing volteer config. So let's define it.

BUG=b:178099673
BRANCH=none
TEST=build zephyr and see that extpower_gpio.c is still built

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: Ica930e9121e041c8159e802c4f86103073410614
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2678788
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

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