ARM: DRA7: Consolidate voltage macros across different SoCs

The voltage values for each voltage domain at an OPP is identical
across all the SoCs in the DRA7 family. The current code defines
one set of macros for DRA75x/DRA74x SoCs and another set for DRA72x
macros. Consolidate both these sets into a single set.

This is done so as to minimize the number of macros used when voltage
values will be added for other OPPs as well.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>