Bluetooth: Controller: nRF54Lx: Use SW_SWITCH_SINGLE_TIMER

Use SW_SWITCH_SINGLE_TIMER for nRF54Lx so that fast ramp
can be used, and hence support assymmetric PHYs in ACL
connections.

nRF54Lx radio domain does not have second timer instance
to efficiently implement software switch unless a second
time is used from a different power domain needed use of
PPIB for the double buffered switch timer compares.

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