ARM: OMAP2+: enable REGULATOR_FIXED_VOLTAGE

Enable REGULATOR_FIXED_VOLTAGE for all OMAP2+ platforms
otherwise system can't boot from SD-card when kernel is
built for single SoC (for example, with CONFIG_SOC_DRA7XX=y only).

It's also required for almost all TI SoC's platforms.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>