kconfig: openthread: Fix PSA Key Ref config for RCP architecture

`OPENTHREAD_PLATFORM_KEY_REF` should not be selected for RCP
architecture, as keys are handled on host's side and passed to
device over Spinel, and therefore they cannot be exported
on platform from reference with PSA Crypto API.

Signed-off-by: Maciej Baczmanski <maciej.baczmanski@nordicsemi.no>