Bluetooth: Controller: llcp: Fix PHY update FSM stale after instant

In peripheral role when:
 - data length update is enabled
 - the CONFIG_BT_MAX_CONN is set to 1
the PHY update control procedure after reaching instant sends two
notifications to host. It notifies host about PHY update and data
length change. Both notifications are send one after another, so two
free node rx are required. The number of available node rx in
provided conditions is one. The PHY update FSM is stalled in waiting
state for enough empty node rx. At the same time remote device is
allowed to send new remote control procedures. Received request are
handled by PHY update FSM that asserts due to unknown procedure opcode.

The commit changes number of allocated nodes to be two times a number
of LLCP connections.

This is a workaround for the issue. The best solution seems to be
sharing nodsx between connections. Though this solution requires
buffering remote control procedures until host notification is done.

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