arm/dt: msm8226: Modify LPM levels

As retention on core and L2 GDHS is not supported, remove the low
power level. Add L2 retention as low power level as a substitue
for L2 GDHS. Also add a property to detect irqs and gpios at mpm
when xo is off.

Change-Id: I427c962d7baf88827a3cdb3fcbddefee6439732e
Signed-off-by: Priyanka Mathur <pmathur@codeaurora.org>