ARM: DRA7XX: Add support for DRA7XX only build

SOC_DRA7XX was under wrong menu within Kconfig file, which prevented
DRA7XX only build. Fixed the kconfig options for this SoC as we are
there. voltage.c needs to be added to the DRA7XX build also, otherwise
DRA7XX only build will fail.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>