apm821xx: align gpio-keys node names with dt-schema

apm821xx: align gpio-keys node names with dt-schema

As per the series:
<https://www.spinics.net/lists/devicetree/msg508906.html>

"Enforce specific naming pattern for children (keys) to narrow the
pattern thus do not match other properties.  This will require all
children to be properly prefixed or suffixed (button, event, switch
or key)."

Signed-off-by: Christian Lamparter <[email protected]>