RPI DTS: Add value previously set by VideoCore and DTB links

Using latest U-Boot for RPI 1 or 2 the DTB loaded by the firmware is discarded.
The DTB was previously patched by the firmware to contain the DMA channel mask.
DTB provided by the rpi firmware or DTS in the Linux tree contain the raw value
directly. Do the same for our DTS as we cannot switch to the upstream ones yet.
Not having the DMA channel mask setup properly cause mmc not to be detected
(and probably other problems on driver using DMA).

Also, add links for rpi dtb to the name used by u-boot. This way the dtb can be
loaded by ubldr using the U-Boot env variable fdtfile.

Tested On: RPI B Rev2, RPI Zero, RPI 2 v1.1 RPI 2 v1.2

Thanks to Sylvain Garrigues <sylvain@sylvaingarrigues.com> for the help.

PR: 218344