bluetooth: controller: Expose prpa cache for LLL use

The prpa cache is now exposed through the ull_filter_lll_prpa_cache_get()
function (as the resolve list already is). This is needed to be
able to reply to AUX_CONNECT_REQ within the required time when
using SW-based RPA resolving

Signed-off-by: Troels Nilsson <trnn@demant.com>