drivers: bluetooth: hci: refactor silabs controller init

Refactor the controller init to use the new init routines the
controller now provides. Instead of using a hardcoded set of
features, the features are now initialized based on Kconfig
values that the application selects.

Add config files under modules/hal_silabs, used by the controller
init routines in the external module hal_silabs to determine which
features to enable based on Kconfig options. This brings the
controller feature configurability to roughly parity with the
Simplicity SDK.

Signed-off-by: Jori Rintahaka <jori.rintahaka@silabs.com>
Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>