modules: hal_nordic: nrfx_config: Customize NRFX_SPIM_EXTENDED_ENABLED

nrfx_config is setting NRFX_SPIM_EXTENDED_ENABLED based on presence of
a DT property. However, there are test cases when it is expected that
extended SPIM features will be disabled on a target that supports
extended features. Allow that by not setting the value if it is already
defined.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>