dts: bindings: Add CPU device bindings for Cortex-R.

This commit adds device bindings for Cortex-R4(F) and Cortex-R5(F).

These were supposed to be added during the initial development of
Cortex-R port, but it was not due to an incorrect device tree
specification.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>