Bluetooth: Controller: Warn LLCP Legacy implementation as DEPRECATED

Add in Kconfig, select DEPRECATED, to warn at build time
that the LLCP legacy implementation is deprecated and the
refactored LLCP enabled by BT_LL_SW_LLCP Kconfig option be
used instead (which is the default now).

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>