Bluetooth: controller: Move DF feat selection to LL_SW_SPLIT KConfig

The BT_CTLR_DF_SUPPORTED KConfig was a wrapper for DF related features
set that are supported by Zephyr's BLE Controller. At the same time the
KConfig is used by Host in compound build to enable BT_DF. That makes
the whole thing a bit coupled with Zephyr's controller.
External implementations of controller must provide same feature set
as Zephyr's Controller or there will be an error during build.

The PR moves the Zephyr's Controller DF supported features set to
Kconfig.ll_sw_split, where it is included only when BT_LL_SW_SPLIT
is selected. BT_CTLR_DF_SUPPORTED is now a top DF_SUPPORTED KConfig
option that enables possibility to select actual DF features.
That allows to select individual set of DF features by any controller
implementation and makes possible to enable BT_DF and build selected
DF Host features.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>