Bluetooth: Controller: Kconfig: Move BT_LL_SW_SPLIT specific configs

These configs are very tied to the BT_LL_SW_SPLIT implementation,
so it makes sense that these are only visible when that link layer is
used.

For the ones that may be used by other controllers in the future,
a dependency has been added.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>