ARM: OMAP2+: avoid NULL pointer dereference

For OMAP4, volt_data is set in omap44xx_voltagedomains_init.
If the SoC is neither OMAP443X or OMAP446X, we end up with a
NULL in volt_data which causes a kernel oops.
This is the case when booting OMAP4470.

Signed-off-by: Nicolae Rosia <Nicolae_Rosia@mentor.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>