OMAP3+ VP: replace transaction done check/clear with VP ops

Replace the VP tranxdone check/clear with helper functions from the
PRM layer.

In the process, remove prm_irqst_* voltage structure fields for IRQ
status checking which are no longer needed.

Since these reads/writes of the IRQ status bits were the only PRM
accesses that were not to VC/VP registers, this allows the rest of the
register accesses in the VC/VP code to use VC/VP specific register
access functions (done in the following patch.)

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