board: dra71: Fix selection of OPPs

As per the DM[1] Dated June 2016–Revised February 2017, Table 5-3,
DRA71 supports the following OPPs for various voltage domains:

VDD_MPU:	OPP_NOM
VDD_CORE:	OPP_NOM
VDD_GPU:	OPP_NOM
VDD_DSPEVE:	OPP_NOM, OPP_HIGH
VDD_IVA:	OPP_NOM, OPP_HIGH

This patch add support for selection of the above OPPs instead of
using OPP_NOM for all voltage domains.

[1] http://www.ti.com/lit/ds/symlink/dra718.pdf



Reported-by: Vishal Mahaveer <vishalm@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>