Bluetooth: HCI: Re-organize vendor read static address handling

Add header definition for bt_read_static_addr function. Declaring it
without a header definition will not give any compilation error when
function definition changes.
Refactor nRF SoC specific code into nRF specific source files and
provide weak definitions when these are not implemented. This will make
it easier to add handlers per vendor.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>