ARM: dts: rmobile: Fix RPC-IF device node names

According to the Generic Names Recommendation in the Devicetree
Specification Release v0.3, and the DT Bindings for the Renesas Reduced
Pin Count Interface, the node name for a Renesas RPC-IF device should be
"spi".  Especially on R-Car Gen3 and RZ/G2, the node name matters, as
the node is enabled by passing a DT fragment from TF-A to U-Boot, and
from U-Boot to subsequent software.

Fix this by renaming the device nodes from "rpc" to "spi".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>