ARM: OMAP: DRA7: Make use of omap_revision information for soc_is* calls

Currently of_machine_is_compatible is used to detect the soc which
employs string comparison operations. We already have all the required
information in the omap_revision. Hence make use of the same like
the previous OMAPs and avoid costly string comparisons.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>