zephyr: Update KEYBOARD_PWRBTN_ASSERTS_KSIx to be a choice
zephyr: Update KEYBOARD_PWRBTN_ASSERTS_KSIx to be a choice

We cannot have both KSI2 and KSI3 selected here, so convert this into a
choice. Add a third choice for the case when neither is selected.

BUG=b:167405015, b:179482109
BRANCH=none
TEST=ninja -C /tmp/z/vol/build-ro/ menuconfig
Check the menus look OK
Build volteer for zephyr and check that we still have
CONFIG_PLATFORM_EC_KEYBOARD_PWRBTN_ASSERTS_KSI2 selected

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: I0a8de0cf8d60e5d87d3d6d102fcf35c4b8fe06f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2678781
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

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