OMAP3+: voltage: remove unneeded debugfs interface

Remove read-only debugfs interface to VP values.  Most of the values
are init-time only and never change.  Current voltage value should be
retreived from the (eventual) regulator framework interface to the
voltage domain.

Fixes to original version provided by Nishanth Menon <nm@ti.com>

Signed-off-by: Kevin Hilman <khilman@ti.com>