ARM: dts: Fix incorrect dcan register mapping for am3, am4 and dra7

We are currently using a wrong register for dcan revision. Although
this is currently only used for detecting the dcan module, let's
fix it to avoid confusion.

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