[DRA7] rename enum to 75X

With DRA72X CPU support, it is confusing to retain DRA_7XX enum to
indicate the 75x SoC. So, rename it to indicate 75X SoC.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Misael Lopez Cruz <misael.lopez@ti.com>
[CPUINFO] [DRA7] support DRA72x SoC

Add support for DRA72x SoC

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Misael Lopez Cruz <misael.lopez@ti.com>