Start cleaning up the voltage layer to have a voltage domain layer
that resembles the structure of the existing clock and power domain
layers.  To that end:

- move the 'struct voltagedomain' out of 'struct omap_vdd_info' to
  become the primary data structure.

- convert any functions taking a pointer to struct omap_vdd_info into
  functions taking a struct voltagedomain pointer.

- convert the register & initialize of voltage domains to look like
  that of powerdomains

- convert omap_voltage_domain_lookup() to voltdm_lookup(), modeled
  after the current powerdomain and clockdomain lookup functions.

- omap_voltage_late_init(): only configure VDD info when
  the vdd_info struct is non-NULL
Signed-off-by: Kevin Hilman <khilman@ti.com>