ARM: OMAP2+: pm: Use hwmod name instead of dev pointer

Replace the struct device parameter of omap2_set_init_voltage
by the hwmod name. It will avoid having to store explicitely
the device pointer into a static variable.

Moreover, it will be a little bit more scalable if we introduce
new DVFS devices.

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