drivers: lpm-levels: Use cpu_do_idle for shallower LPM entry

dsb should be invoked before wfi instruction without which
core can get stuck or do not respond to IPIs. Use cpu_do_idle
that invokes both dsb and wfi instructions for shallower mode
entry.

Change-Id: I2e9d2317048781590a626a0b3d93f23bbf9eb496
Signed-off-by: Raghavendra Kakarla <rkakarla@codeaurora.org>