Bluetooth: controller: Improve mem mngt of proc ctx

Move the mem_pool structure to the internal header for wider access.
Introduce a mem_pool owner in the proc_ctx, sch that multiple mem_pool
could be used for memory management of proc_ctx, also static 'foreign'
allocated proc_ctx would be supported by this.

Signed-off-by: Thomas Ebert Hansen <thoh@oticon.com>