ARM: OMAP2+: pm: Remove static devices variable for mpu, dsp, iva and l3 PM

Since the device pointer is now retrieved using the hwmod name, remove
the static variables used to store the device pointers for DSP, MPU, IVA
and L3 devices for PM/DVFS usage.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>