OMAP3+: VP: move SoC-specific sys clock rate retreival late init

Add sys clock name and rate to struct voltage domain.  SoC specific
voltagedomain init code initializes sys clock name.  After clock
framework is initialized, voltage late init will then use use the
sys_clk rate to calculate the various timing that depend on that rate.

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