Add CONFIG_PS2 to config_allowed
Add CONFIG_PS2 to config_allowed

This option is defined by Zephyr, but the Zephyr source tree is not
always accessible to the script. Add it to the list to avoid any
errors.

BUG=b:181323955
BRANCH=none
TEST=try CQ
Build for primus then check (without a Zephyr tree):
./util/kconfig_check.py -c build/primus/.config
	-a util/config_allowed.txt -p PLATFORM_EC_ -s zephyr/ check
See that it now succeeds

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: I08f39eb5c5e8d1c50aede1e5c1ec888e3d1027f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3451701
Reviewed-by: Zhuohao Lee <916bf7ddbf0565669e59e3cb2b613381a48066c0@chromium.org>
Commit-Queue: Zhuohao Lee <916bf7ddbf0565669e59e3cb2b613381a48066c0@chromium.org>
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Commit-Queue: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

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